This time of year you can expect prices to be about $500 cheaper than in the summer. But at the same time, the supply of bikes goes down as well...
This varies by area but in Massachusetts..
a pregen (07 and prior) will run about $1,000.
A newgen (08-12) will run you $2,000.
A 300 will run about $3,500.
(winter prices)
KBB prices generally are not a good indicator for bikes, just find a bike you like and link it here and we'll let you know what a fair price would be for it.
Like Katie said, mods don't increase value but pay attention to what mods they post. an aftermarket seat is always nice