In answer to your question I would estimate that I spend about £10 every second week. Taking into account bugs, substrate and other bits and bobs. But I am near the cheapest shop so I guess if you allow £15 - £20 every two weeks I would think that will cover it.
There are a lot of care sheets available on here, if you can't find one 'sleepydee Geckos' website wil have one and its good. They wil generally tell you wwhat you need and then it's just a case of shopping round. The cheap shop I mention is online and is called Surrey Pet Supplies' check them out as you may get stuff cheaper.
Good luck with it all and don't be afraid to ask questions on here even if it seems silly.
1.0.0 Royal Python
0.2.0 Leopard Gecko
0.1.0 Jack Russell Terrorist