Buy - there's not much to be gained by building.
You can buy boxes at the right time of year from Mann Lake for about $6.50 a piece - unless you can get 1x stock really cheap, you're gonna have a tough time breaking even at that cost. You can build with 2x stock, and probably get down to about $4.50 a box, but then you've got nonstandard equipment. Things like Nucs, mating boxes, etc, can probably be made from 2x stock without getting into too much trouble (and they tend to be overpriced in catalogs).
You can legitimately save some money building frames - if you just take wood costs into consideration, and ignore labor and tool wear. You can make a couple dozen frames from a $15 2x12x12 - but you're going to spend a ton of time, you're going to put a lot of wear on your router and table saw, and you're going to need to make really precise cuts.
Bottom boards, inner covers and telescoping covers are a pretty good place to make some cash back though - they're way overpriced, and pretty easy to build. Stick to building the "I need one or two of these" items and you'll be fine. Buy stuff that you need in bulk.