A lot would depend on the caliber you are shooting. if you shoot round balls from a .58 caliber rifle, you might get 25-26 balls from a pound of lead. For a .50 caliber rifle you might get twice as many balls from the same amount of lead. You could probably get set up with basic casting equipment (Lee mould, iron pot and dipper) for around $100. Swaged or cast balls appear to be going for around $15-$20 per box, with 100 balls of the smaller sizes or 50 of the larger ones per box. I haven't priced raw lead recently because I have enough to keep me going for a while (thanks,
@Justin.44 !), but I think it's running around $2-$3 per pound. So, if you sell your lead, you might get $70-$100. That won't really buy very many pre-cast or swaged balls.
Your time may be a factor, also. However, most people who cast their own enjoy the process, more or less, and their time spent casting is recreational... Part of the muzzleloading experience. If you find it's something you don't like doing, that may influence your decision.
Casting round balls is easy. Big, hollow-based conicals might be more challenging and the moulds for these cost more than round ball moulds, but big, hollow-based conicals are also a lot more expensive to buy. So, you also need to consider the type of bullets you'll be shooting.
For shooting just a few rounds occasionally, store-bought bullets may be more cost-effective, but with the amount you are shooting, I honestly think you will save money if you get set up to cast your own.
I hope that helps! When the boys on the forum see your question, you'll probably get a lot of encouragement to start casting.
Notchy Bob