Wait for the markdown sale. Last year I picked up a large weeping one at HD for less than $30. Keep checking because both HD and Lowe's marked them down along with all their trees in June.

Metro Maples in FW has pretty reasonable prices and a good selection.

Here is a link that might be useful: Metro Maples

I've bought some from Lowes, and a few other Austin stores. I actually saw some 5 ft seiryu at Costco for $100 the other day. I'd wait a while for a sale. The most I've paid for a Japanese maple was $70. They have all been in the ground for 1 year now and are really pushing out new growth well (Rased bed on East side of house). So even if you buy a smaller one, it won't take as long as you think before they are big enough to take your breath away.