In general, people who do this kind of SEO are a bit, um, scummy. There are exceptions surely.
That said, $6000 for a reasonable size custom website (ACTUALLY custom, not just "I loaded up wordpress and put a decent template I already had laying around on it") is not bad. I've been known to make real custom websites for people, and even 10-20 pages can easily hit that price. Again, this is actually made-to-order custom does-everything-you-want, not "I loaded up Joomla and put a template on it that you kinda-sorta like."
If they are then guaranteeing you first position on Google for a set of keywords (they can't do this, but could offer e.g. a refund if they fail), that's worth something potentially. Make sure you know up front what search strings they're "guaranteeing" 1st position on and that they're strings you actually care about. Quotes matter, etc.
Note that Google et.al. take a pretty dim view of things like this. If the SEO guys play any "tricks" like linkfarming, etc., Google may well just delist the entire domain, potentially in perpetuity. Make sure the domain isn't particularly important long-term.
Also, make sure you're also not signing up for a spam email campaign. This will tend to just piss your customers off rather than attract them. Well, unless you're actually trying to sell Viagra online, that is.
Make sure the hosting is decent, too. Yes, Hostgator and whatnot can host you for $2.50/mo. No, it's not very good. Compare with what actual hosting service you're getting out of this. It doesn't do you any good to be #1 on Google for something if your host can't handle the corresponding traffic.
You're dealing with the seedy underbelly of the INTERNET, here. Dot all the Is, cross all the Ts, and make sure your recourse is clearly defined. If these guys aren't in the same country, assume you will be scammed with no recourse. That may not actually be the case, but the risk is just too great, IMO.