I am a BIG DIYer myself, so I am Subscribed! It is great to reuse parts, isn't it. Nothing better than free!!!
Man, I didn't realize that retesting a tank is so high! I wonder what is involved in retesting a CO2 tank?
Anyway Welcome, and I can't wait to see what Ya do next!
Mostly they check for cracks, or damage that could cause a rupture of the tank. Just had a propane tank for a forklift certified, $15 at a propane place (they made us sand it down and repaint it, before they would stamp it). My 5 and 10 lbs co2 tanks get re-certified for free from the place I bought them from, as long as I keep their sticker on the tanks. Otherwise I think they said $20 to certify.
If you were at paintball place for the recert, try going to a welding supply shop, they might be able to re-certify it for cheaper, then you will have a backup tank, instead of a paperweight.