The easiest way to get an idea of how much those tapes are worth would be to search for them in the "completed" sales listings on eBay. That will tell you what somebody actually paid.

As a general rule, the 7-1/2 IPS tapes are worth more than the 3-3/4 IPS versions, but that's a relative thing. If some collector wants the particular tape you have, they might pay $35-50 for a Beatles tape -- probably a bit less for other bands that are not as widely collected. Of course, there might be some extremely rare titles that are worth more, but don't count on it.