for one, a 2ft by 1ft by 1ft tank would be great - glass or wood, though glass lets more heat out than wood. you could buy it or make it yourself easily enough. if you buy it they are £100 - £200, depending on the material and where you get it from.
a basking light too, i use a 60watt clear bulb - these are about £3.
and the fixture to attach it into the tank aswell are about £6.
a water bowl can be bought for about £2, or up to £15, depending on the size and material its made from. you could always just use a smallish container you already own though.
papertowels for substrate is good to use and practically dont cost a thing! the geckos also cant swallow it and cause damage to itself.
an under tank heater, thats about half the length of the tank, and goes on the same side as the bulb. these are usually about £7 - £10.
then hides. you could use a cardboard with a box with a hole in it, or something similar, or you could buy a specially made reptile hide, which are about £4. you should have one for each gecko you have and have one in the hot end of the tank and one in the cool end. when it sheds, you should also put some moist reptile moss (£2 for a bag) or something in one of the hides to help it to shed properly.
cork bark is good too. you can find it and wash it well, but its easier to buy it haha. its about £3 for a piece, and you can get all sorts of shapes and sizes that make your tank look nice, and the gecko with rub on it to help it shed.
fake plants look nice too and are about £3, but the lizards arent really bothered about them - the bugs just seem to hide in the one i have.
i also have a flat stone in my tank, under the basing bulb. you dont need one but it looks nice :) they are also about £2 depending on size and stuff, or you could just go find one and clean it.
of course, you also need a tub of calcium power (£4) to dust the food with. they can eat all sorts of bugs, no bigger than its head, and should be fed on more crickets than mealworms, as mealworms can cause damage in large ammounts. food will be about £2 a week :)
no idea about crested geckos, sorry - they look nice though :D
i depends on what you want from a pet that makes it good.
good luck though!