VirtualGareth: Is there really much of a difference with the ones you have to pay for?
Obviously there is a difference, the only reason to pay less or more for custom balls. Much of a difference? That depends on the balls and on the clubs.
Major difference from the cheapest ball up is the "feel" - meter speed. Every custom ball has a slower meter than the Starters.
Second general difference is precision IMHO - even with Starter clubs you get better shot results. These two together - less deviation from ding and higher precision - are essential for hitting greens instead of missing them.
Third is the spin effect. Custom balls have different spin dots, but they all have more spin than Starters.
Fourth is distance with equal shots. Though I don't like it, it can be essential for top players if they can pass a hazard, reach a green with a shot less or with better precision from a closer lie.
I'm sure there are more advantages...
On the downside, the price. Custom balls eat credits constantly. Ball life is limited (90-125 hits), and balls get lost. Cheap balls may be equivalent to 1 credit per 9 hole game (stroke and matchplay), the SDs are about 6 crs, Callies 25(?) and top Nikes 50 crs or more.
Furtheron, sometimes I experience what a few players report here, the meter may show less reliability (more stutters) for higher quality balls.