Best budget mattress for a child?

[Specificity]

We have been using simple foam ikea ones but they need replacing and I'm shocked at the price of them, need them cheap as possible (3 to buy) but the one we have from local bed shop (cheapest single) is alwful, you can feel the springs.

Any suggestions?

John Lewis do a Value range. Good quality.

I've always bought memory foam ones from eBay. Think I pay £80 for the single ones.
Kids give them the thumbs up.