As for the filtered part I'll add my tuppence worth. If you want to do it (relatively) cheap, but give him a good home here's what you need (but don't take my word on it lol, I like to spoil my fish and give them loadsa space. Would you say 60-80L is enough for a male

):
50 watt heater or 25W depending on size of tank = £13 maximum
Air Driven filter (Algarde biofoam 45) + air pump = £10 max
Gravel (again depending on size of tank) = £2-£5
Plants (real if possible as plastics tend to tear betta fins

) £2-£5
+ lil fishy friend (betta) = £3 - £20? I'm not too sure what a halfmoon would cost, but they tend to get more expensive as the finnage get more fancy.
Oh and tank. 20 - 30L should be about right. Pick one up 2nd hand, give a good scrub. Cheap as chips!
So max = £49- £55 (not cheap cheap)
Min = £35 -£40 approx
It's still a small bit of cash to part with, but would be a good start. I'm sure you could do it cheaper, if not go el natural with no heater or filter. Just make sure he's nice and warm.