Hey Josh
How old is the crested gecko going to be ?
If you buy a baby you just wanna buy a medium size faunarium until its about 6 months old
once its older then u can move it up into whatever size tank you want.
What i'd suggest is getting a baby and buying a faunarium costs about £5:50 or so, and if you want to buy a tank, buy a big exo terra, tank then as an adult it will live comfortably, and if it turns out female you can comfortably add more.
Now as you don't want to over spend your tank won't look as amazing as it could but
A basic but very fuctional set up would be for a baby
A few plants for them to hide in, 2 milk bottle tops one for the crested gecko diet and one for water if needs be but spraying should provide sufficient water for the crestie to drink.
that set up will really cost you nothing, just make sure u can keep the temperatures right!
If your getting an older gecko, you will need more plants, more vines for them to climb from side to side, and will be more expensive but you can do the exact same as cheap water food and water bowls, just use tea light candle holders. which are very inexpensive.
As in my opinion, having a great enclosure with great hides and coverage is much more beneficial then fancy looking food/water bowls until you can afford to make the finishing touches

.