Polythene framed greenhouses

I bought a polythene type greenhouse with snap-to framework which I picked up from Aldi two years ago for £30.  The polythene has lasted two years but the framework and shelving is still good for quite a few years yet.  Dimensions are 5 feet wide by 6 tall (sloping) by 2.5 deep.  It has been invaluable in starting many plants off early.

My questions:

Would it be more cost effective to buy a more expensive model - are there models with more photo-stable polythenes?

Would it make more sense to find a model with replacement polythene covers; but I presume they would be more expensive.

Is £15 per year a reasonable outlay? I haven't got room for a glass and frame greenhouse and not sure one would pay for itself.  What do other folks do?

Thanks for that. Mine came with a clear cover and a clear cover reinforced with nylon grids.

I think the clear one may have been to increase the warmth inside rather than as an outside cover, but I was concerned the reinforced cover might not be letting in enough light - seedlings seemed drawn.

I'll try and find a suitable replacement as the framework is still ok.

Found a reasonable deal on eBay as suggested- thanks