For under $100, I recommend the Hagerman Bugle http://www.hagtech.com/bugle.html#bugle. You buy a blank circuit board ($35) and the parts ($30) from Digikey, which you solder on to the board. Ideally you should have some soldering exprience, but I built one without any electronics knowledge or soldering experience. You can also buy it for $150 fully assembled.
The disadvantage of the Bugle is that it requires two 9-volt batteries that only last about 16 hours. To fix this, you can add a power supply that Jim Hagerman sells, switch to more efficient (but lower fidelity opamps), or use a longer lasting lead acid battery (as recommended by John Elison on audioasylum) http://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=342009&highlight=+elison+lead+battery&r=&session=.
I compared the Bugle to the NAD PP-2 ($129 retail), and liked the Bugle better.
The disadvantage of the Bugle is that it requires two 9-volt batteries that only last about 16 hours. To fix this, you can add a power supply that Jim Hagerman sells, switch to more efficient (but lower fidelity opamps), or use a longer lasting lead acid battery (as recommended by John Elison on audioasylum) http://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=342009&highlight=+elison+lead+battery&r=&session=.
I compared the Bugle to the NAD PP-2 ($129 retail), and liked the Bugle better.