A Fulltone OCD (like the used one I bought recently) can give you over 20dB of clean boost, and can give you overdrive as well. They retail new for about $150 USD - I think that's a bit over your limit but still worth considering (especially if you can find a used one.)
There are tons of other gain boosters around, too but I can't claim any experience with 'em.
And, like the above posters say, most overdrive pedals can also serve as a clean boost if you turn the drive all the way down and just use the level knob. I guess the same goes for most pedals that have a level knob.
I've got a Lovekraft Pusher that's a great clean boost. That's all it does, too, so i don't have to waste an actual OD pedal on a boost. Of course, just about any clean boost can be used to overdrive an amp.