the CM16 carbine is around £100 if you can find one
They do come up on offer for around £100 occasionally but I can't see anyone selling it for that at the moment.
The minimum package I can see is at
https://www.swasairsoft.co.uk/ , who will do it for £120 + £5 for two toning + (bare minimum) burn-your-house down charger and an 8.4V 1600 nimh for another £20, with postage on top so you're north of £150.
Other combos are available, and I'd be wanting to buy a pair of lipos from HobbyKing plus a charger. Note that these cheap lipo chargers also run very hot and are likely to result in tears before bedtime (and for the love of God never leave a battery charging while you're not around).
The G&Gs are better than the CYMAs, but I can confirm that CYMA M4s (and doubtless the AKs) are actually perfectly usable toy guns.
If your budget is really tight, you
can get playing with one of the two-toned CYMAs for £90 + postage from either PatrolBase or SWAS. A hi-cap, battery and death-charger is included, and you can always upgrade later when budget allows.
Normally I'd recommend SWAS, but on pure aesthetics, the PatrolBase burgandy red is a nicer colour than orange or green as offered by SWAS.
Oh and inB4 "which G&G CM16 / CYMA M4 though?", the answer is: whichever one you think looks the best.