Minimum that you want to spend on a mask is around £40 unless you're buying used. The masks that I would describe as good start at about £50.
Which mask is best can differ from person to person based on the size of someone's head, the way they breathe, they're hair, whether they wear glasses and all sorts of other things, the best thing I can recommend is to go to a local shop or ask players at your field if you can try on their masks to get a feel for how they sit on your head.
You can get a pair of JT Pro-flexes which are really popular or a V-Force Profiler for around £50, those are probably the better ones I can think of at that much.
Raise your budget to around £75 and you'll get a lot more options but those are my recommendations for spending as little as possible