$100 is kinda tough for a full size reg and a 2.5/5# tank or so, not to mention the other misc things you'd need. I have an Aquatek mini paintball reg on my 29g and a full tank will last about 3-4 months before needing refilling. You could do that one plus a 20-24 oz paintball canister for around $110-$120. A full size reg with tank would prob be a little more than that, but you can also find good deals on Craigslist and such (particularly for the co2 tanks). Aquatek is overall a good brand for budgeted co2 regulators/equipment.