Cheap underlay or a bad idea?

I bought some laminate flooring at b&q to lay in my new home which I recently moved into. Previously I was in one room, so now I'm having to completely do up and furnish a one bed bungalow from scratch and it's costing a packet, so naturally I need to save every penny I possibly can. I was wondering if I could possibly save a bob or two on the underlay for my laminate flooring by using, cardboard cartons, opened up and laid flat and carefully taped together. My existing floor is wooden floorboards over a 15-18" empty space. To be quite honest, cardboard boxes, once they are opened right up and laid flat, do seem to be much sturdier and hard wearing than the flimsey stuff you buy on the roll, unless you go for the really expensive stuff that's almost as much as the flooring itself.
Can anyone possibly give me a reason why I shouldn't do this. Although I want to save money where I can, I dont want to bugger the job up just to save a bit.
I've no time limit on the work I'm doing, so I will not go ahead until I get a second opinion.

Thank you for reading this, do you think it would be alright?

It would have the effect of an underlay but would wear in time and if it ever got wet it would be a lot more of a problem than underlay.

I`m with DAZB - No!

........the correct underlay isn`t that expensive

Thanks for replying to my question about laminate underlay. I will take on board the advice and forget my idea. Thanks for both replies.

The thin flimsy stuff on a roll is useless. I've been to friends houses and commercial installs where they've used it and they all have the same problems: there's not enough cushioning so the laminate ends up flexing a lot. It just feels like a cheap job, which is exactly what it is. Not nice.

The stuff I've used at home is far better. It's fibre board. The stuff is 7mm thick sheets. Price works out at around £2 per square metre. It's easy to work with. Doesn't move around like the foam tissue, and the finished flooring job lies flatter and there's none of the "hollow underneath" springiness that ruins the effect of a nice laminate job.

The only time I'd use the thin foam stuff is as a damp-proof membrane over concrete floors before laying fibre board.