Honestly, I'm way out of my knowledge base when it comes to how the DoD and Big Army solicit contracts. What I have seen is a buttload of waste on big ticket items, while basic kit and armament is cobbled together from spare parts or is non/barely-functional. After seeing so much waste and mismanagement, I think the DoD and Big Army can be better at leading and equipping the Infantry.
Example -
DoD/supply/etc : Heya grunts! Here are 100 MRAPS. Oh... but we can't do much about the cobbled together 240's, SAW's with warped receivers, non-functional lasers, worn out gloves/pants, etc. If you're hungry we have steaks, lobster, and artisanal coffee at the nearest FOB though.
Grunt: Uh...
DoD/supply/ect : Also, please stop cavorting off with our supply of sundries and snacks during your visits. We know we've been feeding you Pop-Tarts and expired Pancake Heater Meals, but nevertheless please behave like civilized human beings and respect our property.
Here's how the acquisition process works (just a real high level overveiw).
Each year on Oct 1, they receive their annual budget. That budget includes everything they need for the year, to include payroll, beans, bullets, and bandaids. That money comes in five different "colors"; Operations and Maintenance (O&M), MILCON, procurement (proc), R&D, or MILPERS.
To your example, MRAPS are most likely bought via PROC funding (no idea off the top of my head what one costs but they are probably higher than $200k). SAW's, lasers, sights, etc are O&M. It's against the FMR to use one color of funding for something other than what it was intended for. So if your command requested new MRAPS, they'll be given money for those. You also only have finite funds available as a commander and you are trying to ensure your commanders guidance is met through the funds you are told you have. Some of the higher grade officers around here can attest more to that. In my current position as a Contracting Officer I just make sure the funds are available when releasing an award. I have no input on why I'm buying something or how much money is left available. Unfortunately for most of us at the lower levels who are grunts living and working in the barracks and rarely around the head shed, we never know how the supply side works as we aren't exposed to it.