It's not just the cost of the paint you have to consider. In the UK it has for a very long time been a "What is the cheapest paint we can get" This is all well and good, if you don't give a sh1t about the quality then fine. In the states they look at it a little deeper. You can get your every day case of paint for less that 40 bucks but it's just cheap paint, but you can also get 100 dollar cases and that is what you see being shot at the tourneys you pay top whack for good paint. We (Nexus) are very lucky and we get a great deal on paint so when we are in the states we shoot Ultimate Evil (RPS) which is unbelievable, i have NEVER seen a paint perform the same as this in the UK.
If you want to train the sometimes hard or brittle paint can be good, focuses the mind when you know you need to drill the guy 5 times before one will break and if it's brittle it will teach you to speed squeegie in seconds, no good not knowing how to squeegie under pressure is you only train with hard paint! Top notch tourney paint is going to have the odd burst.
So the question cannot be answered in a pounds and dollars figure, it depends on what you want your paint to do, If it's shoot at 299fps every ball, always break on impact, never break in your barrel, Mark like hell, shoot like darts and cost you 20 pounds a case then i think it's time to wake up !!