Pretty difficult to estimate a value for the gun having seen only one side of it. How badly rusted is the RHS?
The box would need a great deal of TLC to restore, but a lot depends on what the top label looks like.
The two barrels, presumably .177 and .22 is a bit of a plus - provided the bores are not rusted.
It would make a decent collector/shooter if properly restored/reblued.
A few more pics would be most helpful.
It would definitely be worth more properly restored.