Reloading costs only a few more pence/shot than RG (the cost per case becomes negligible once reused several times). But as soon as you factor in the cost of your time the price of reloading is very expensive.
The reasons I reload are many, the primary being accuracy/consistency over shot bought ammo (even the "hand-made" ammo of some dealers!), and a close second being function (two of my competition rifles will not cycle RG ammo at all and the factory ammo is just as, or more, expensive as reloading).
But As ACR said, when the targets are big (as in ETR etc) then I'll use the carbine and RG, it works well enough in that rifle and is cheap as chips. It's inaccuracies are not as bad as many make out.
So to summarise;
PR at matches upto 300 or ETR, RG
PR match with stages over 300, 77MKs
SR matches, 77MKs across the course (I try not to bring two types of ammo to the match as I've fucked up once before and it'll not happen again - until....)
High power, 77MK and possible 80MKs (reduced distance matches - whatever suits, 69's, 77's even 52's on occation, but must be reloads unless I bring a different rifle)
Anything over 600yds, 80JLKs while I still have some, or a big boy's gun.
You can reload for about £20/100
per 100 rounds;
powder (25gn/load, £20/lb) - £7.14
primers (£17/1000) - £1.70
brass (£50/1000, assume life of at least 5 loadings) - £1.00
bullets (£110/1000) - £11.00
Total - £20.84/100 rounds
With RG at an average of £18-20 per 100 it really is not cost that should make you buy RG, rather the lack of time or if you might lose brass
Add in labour at (I'm doing it as a favour to myself) - £10/hr
total = £30/100 - start buying factory ammo.