How do coaches have a lot of recruiting money by saving every year? I had $30k to save and now it's $7k. I thought we got to keep half of whatever's left?

You only get to keep 25%; used to be a lot larger years ago.

Yeah it's 25% and to be honest I'm not sure how coaches have war chests either. Even at 25% it's arguably trivial especially after the first year. Unless you always find recruits for 400$ I don't see it as a viable long term plan. I understand backing off one year to save enough for a battle the next year but after that it's pointless in my opinion. True "bonus" money comes from playoffs/bowls. Not carry-over.

Wasn't it a large sum a few seasons ago? I remember reading don't go to war with people with a lot of time at a school because they can spend 6 figures for one player if they have to battle.

That is because of bowl/playoff money. If you are in a good 1A conference, for instance, you may end up with 80K in bowl money in addition to your normal scholarship allotment.

In D3, there is money from five rounds of playoffs. There are also are some coaches no one bothers to challenge. Over time, that 25 percent can accumulate into usable money for their (infrequent) battles.