I think you will be just fine with less expensive surround speakers. If you're mounting the surrounds on the side walls, it's almost impossible to timbre match these speakers to the fronts anyway. In my last theater, I had the full Revel Ultima line of speakers, and even for this highly regarded $20K+ speaker system, the surrounds still sounded very different than the fronts. Did this detract from the movie experience? I never noticed it.
For movies, the vast majority of surround effects are mixed in independently of the front channel information anyway. Occasionally, you'll have sounds that move between the surrounds and front speakers, but this is not the norm. With multi-channel music, you may notice it a little more, but I don't think it will make that much difference.
In my new theater, I am using Revel C50/F50s in the front, but relatively inexpensive B&W speakers for the surrounds (sides and rears).
If the timbre mismatch really bugs you, you can always add a parametric equalizer for your surround channels (assuming you're using a separate amp). Rane makes some pretty decent ones and you'd probably get a better timbre match this way than using the so-called matched surround speakers.
If you really want to have optimal timbre matching, the only way to do it is to use the same speakers for all speaker positions and make sure that they are placed in approximately the same position relative to room boundaries.
