SSDs for both OS and WoW are going to cost a bit. The larger the SSD the better it performs (up to a certain point) so, 256GB > 128GB > 64GB, etc (in terms of performance). This is a general rule and someone can probably find benchmarks that don't line up 100% but, if you compare the same manufacturer/model and use different size drives... it shows. You can view the differences here, here, and here (do note that some of those benchmarks listed are "lower is better").

With being said, I'll recommend 128GB SSDs but people get by with 64GB. As an example:

2x Crucial M4 64GB = ~$210
2x Samsung 830 64GB = ~210

2x Crucial M4 128GB = ~350
2x Samsung 830 128GB = ~$370
2x Intel 510 128GB = ~$480

As for 10 boxing... you mentioned to me before that you wanted to use video settings better then low on your main. If you want to use DX11 you'll most likely want more than 1.5GB of video RAM. Using DX9 gimps your eye candy but requires < 1.5GB of video RAM. A card that is powerful enough to 10 box smoothly with more than 1.5GB of VRAM will most likely run you at least $500 and that's one-third of your budget. So, with two 64GB SSDs and a beefy video card for DX11 (10 accounts) we've used up about 50% of the budget already.

With what I've posted here so far... what are your (or anyone else's) thoughts?