I would say around at least $2.5k - $3k for a quality site. A website with $100/mo income for sale under $2k is questionable (only if the seller don't need urgent money).If you're looking for sites that are already making money, try Flippa.
Make sure you dig deep to verify income claims
$100 a month blog should set you back around $1500 to $2K or a bit more