If you stake/bet £200 and win £1000 your return is 5 x stake. Doesn't matter if its a single spin or a bonus buy.
If I put a £5 yankee on the horses (£55) and it returns £50 I say I lost £5, I don't think, get in, 10x my stake back. Not sure how it can be calculated any other way.
If you buy a £2 stake bonus for £200 and you get £100 back, do you think you lost £100 or won 50x?