Once again, I'm not sure where you get your information, but just did a quick price check and Vista Ultimate edition is only $280, not $500. And I didn't need to fudge the price since its the full top of line retail version of the OS. I see that OSX is only $115, so it is significantly less expensive, but when you add in that you'll be buying it 3 or 4 times for each one time you buy a windows OS, it adds up to a fair amount more.
I didn't check recent prices, I was going off of intial release. I've not kept up with microsoft's price as it's always a bit tricky to find the "good deal" what with their tiered pricing scheme. I remember finding out that Vista was $499 at release (which I didn't bother to look much further after that either), that's what I was going off of. IIRC the ultimate edition could have been had at OEM prices for $349 or so at release if you got it from the right manufacturer with a HDD. Again, that's just what I remember, but I believe the standard price was $499 retail for the ultimate (i.e. everything) version. Please, correct me if I'm wrong here, it looks like as far as current prices I am incorrect. I'm not trying to mislead here, just state the differences. The different release cycles are not as black and white as people like to think.
Problem with Mac is that most developpers hop on to the next OS very quick. 10.3 or 10.4 are very stable and very good OS-es. Leopard is more eye-candy than it ads really usefull new stuff. And even if you want to support Leopard-features, it does not mean you have to trash 10.4 or 10.3 support as a developer. Yet many do this. Guess >50% of all new software for OSX is Leopard only. It's horrible. Some good exceptions though. With little effort you can both support Time-Machine and other Leopard stuff yet make the program work on 10.2 Jaguar.
With Windows, software developpers support a way longer lifespan of OS-es. Many new programs still work on Windows 2000! That is a 8-year old system. Try to find support like that on a mac. So not only OS-upgrades are cheaper on a PC, all your software is cheaper on a PC, as you can work longer with older products, both old programs on new OS-es and new programs on old OS-es. 10.3 is from 2004 yet almost no new software runs on it! Don't even think about using modern stuff on 10.2 (2003) 10.1 (2002) or 10.0 (2001).
Fun exception, Macromedia Freehand 10 for example was ported to Cocoa immediately with the release of OSX. The program still runs pretty good on Tiger and Leopard. Adobe CS on the other hand still uses Carbon code which has origins in OS 9 which gives way more compatibility issues. Some open source developers still support 10.2 and up. Support for 10.0 and 10.1 is not needed BTW. These releases were very immature OSX-es. Since XP an Apple user was in a way forced to spend 3x129$ on new OS-es. You can buy pretty much Vista for that (especially if you take into account the low OEM price you pay if you buy a new DRAM module along with Vista).
From a developer standpoint there are a number of reasons for this happening. Some of it is laziness on the developers part, some of it is apple's fault. What it boils down to is that the way apple would prefer to go development-wise they can't just jump to because all of the developers would leave. Adobe and some others actually pushed apple into staying with some of their older APIs (carbon) longer than intended. (This is somewhat hearsay, but that seems to be the feeling in the dev community, true or not) If Apple had had their way carbon wouldn't have existed, or would have existed for a very short time for the transition to OS X. However without carbon OS X would have died out of the gate. They've taken the longer and harder road of incrementally doing away with all of their old stuff while trying to stay backwards compatible so that the developers could keep up with the new changes and stay in business. For the uninformed, there are two major APIs to develop for in OS X, the carbon set of APIs (C/C++ based) and cocoa APIs (Objective-C based).
This has it's own problems as you've listed above -- new developers jump right in with current APIs which have just come out. Older developers sometimes abandon their old code once it's rewritten for the new APIs. And then there's companies like mine which are 99% carbon based, still using OS 9 events and will be in real trouble once apple drops this old tech (when they get rid of rosetta) come 10.7 or 10.8.

For the curious, Apple generally does under the hood/developer changes in the even releases and the glitzy user stuff in the odd releases. 10.2 and 10.4 both had major API changes and you could see that reflected in the program requirements. The current developer tools don't even install support for building things less than 10.4, and it's an optional install for 10.3.9 (which is my company's minimum OS version). 10.6 will also be more of this. The current build they handed out at apple's developer conference last week only ran on intel machines, none of their older G4/G5 PPC machines will run it currently. They're moving forward, but it's got it's issues. Better that than not being able to do it though.
Oh, and to keep things on topic, if your primary goal is to do non-gaming stuff and video editing, I'd check out the video editing software that comes with a mac from your brother. It sounds like you're already familiar with the PC way of doing it, so just get informed on the programs taht are out there to do it and have a look. Both a PC and mac can do the job (and depending on your price both can suck or do it well), so it's just personal preference. We can't make this decision for you.

Good luck.