Not to nitpick, (oh who am I kidding?) but the GW tray's are actually the cheapest movement trays for the game. 
Very true, they are surprisingly cheap by GW standards.

The problem is the sheer number that will be needed to play a large game and after all, the whole point is to play with as many figures as possible, after being limited to relatively small numbers with the original LOTR rules.
At £2.45 GBP to base six cavalry figures, it would cost me about £18.00 to base all my Rohirrim, let alone Warg riders, Gondoreans etc. Add to that the far more numerous infantry figures that need basing and you can see it would be a considerable investment. Things are slightly better with the infantry, as you get to base 24 figures for £2.45. Then there is the added cost of paint and flock. Again, it might seem insignificant, but with the number needed it would all add to the burden - and mean less to be spent on the toys themselves. Lastly, the time factor. It takes me far longer to prime, paint and flock models than it does to make card constructs. I recently made a whole card African village in the time it would take to paint and flock bases for 40 tribesmen

One other minus point is that they are designed to work specifically with GW's slottas, but the card versions are generic so I can use them with any other rules system or period where the figures are on 25mm bases - round or square.