Carbon bottles aren't cheap, your looking at £80+ second hand and £130+ new. You may want to look at a stee onel there £45 new and you can pick them up £25+ second hand. There a very good start.
Liam said not to upgrade to get 30bps, he's right, minis can be a right air hog at high rates of fire atleast mine was, ramping 15bps emptied a bottle very quickly.