rule of thumb would be 1.5 to 3 times the annual renewals
.
Depending on the type of business, that can be VERY generous.
Med supp biz might capture upwards of 3x, or some P&C lines but under 65 health lines will be lucky to get 1.5x.
There are companies that will buy a book of renewals. They might have an idea. If you find an agency willing to sell, tell them to get an appraisal by one of these companies. I can't think of the names of these companies right now, but they advertise in trade magazines. You can also try a Google search.