Worth it to replace a freezer gasket?
I replaced gaskets in my side-by-side (so two gaskets at one time, each in the $100 range, IIRC) a few years ago. Easy to do. Ridiculously expensive - my guess is that these are parts the manufacturers get in quantity for literally just a few $ each, but that is the case with a lot of replacement parts, and gaskets are a problem because they have to match the exact size - can't cut to fit.
In the end, comes down to age of the refrigerator/freezer because:
1 - The older it is, the more (relatively speaking) efficient a new appliance will be, so that the savings in energy costs will help to pay for the appliance.
2 - The older it is, the more likely that something else will break. Depending on the part, that may be something inexpensive and easy, or may be prohibitively expensive.
The brand/model has some bearing on this too. The equation on a Sub-Zero (if you would replace it with a similar appliance) is quite different from a run-of-the-mill GE or Whirlpool.