Depends on the model of course, and the condition of both the gun and the box,. Reviewing actual completed auctions I come up with the range of $300-$500 with the guns in excellent condition and the box going towards the higher end of that range and the run-of-the-mill guns no box towards the lower end. Some $600 starting auctions didn't catch a bid.
It seems the variant makes a great deal of difference. Nickel Double-Nines seem to fetch a good price. Some of the other less common ones do also so there must be some collector interest. There were a few different variations of the Sentinel. Add in three finishes, at least three barrel lengths, .22lr or .22 magnum. The 4" was most common, as was .22lr, in a blued finish. So a 2" satin nickel .22 magnum should be worth more
High Standard made quite a few variations in models, barrel lengths, and finishes so if yours is a less common one it's at that higher end.