That's a little bit of a fallacy. Since the KE72-T has 72 inputs and optical inputs, you'd need to buy an IPAC4 plus and OptiPAC to get the same functionality. And once you factor in the shipping costs (assuming that you live in the USA), that difference is probably a lot smaller than you think.
Actually, that's not true, the difference is NOT a lot smaller than you would think - Assuming all you want is the inputs and optical controls - these are the options, by cost.
I-PAC/4 with shipping + Oscar Mouse hack (shipping not included) - $69+$12+10 = $91 - This is only 56 inputs against the 72, but you get USB capability, screw terminals, shift functions, LED support (can be added to the KE-72, but it costs extra)., etc.
I-PAC/4 plus Opti-pac plus Shipping - $69+$44+$12 = $125, keep in mind that in addition to the advantages above, you now have USB or serial connections for two trackballs as opposed to one for the KE-72, you also have the circuitry to auto-switch between the spinner and trackball.
I-PAC/4 plus I-PAC/2 + Shipping + Oscar Mouse hack - $69 + $43 + $12 + $10 = $134 I included this just so no one could say "but Hagstrom still gives you more inputs". I expected it to be more than the KE-72T! - 84 inputs vs. 72 plus all the advantages above.
Hagstrom KE-72T - $139 plus shipping ?? - It's a good product, but really it's only advantage for BYOAC use is the 16 additional inputs on one interface.
I-PAC/4 plus I-PAC/2 + Opti-pac + shipping - $69+$43+$44+$12 = $168 - Just to stay with single vendor solutions
Hagstrom KE-72 with two breakout boards so you have screw terminals - $139 + $16 + $16 = $171 plus shipping, and you're back to "only" one trackball and 72 inputs.
Again, not knocking the KE-72, but no way it's the best value, not even close!
And I didn't even take into account some of the possibilities like the Mini-PAC plus I-PAC/4 or the I-PAC/4 plus KeyWiz plus Opti-PAC, but you get the idea . . .