Squash seed is often a problem when you're buying seed packets. Often the germination rate is low because they harvest too soon to fulfill their contracts.
I have saved seed from 'store bought' squash, but then the other problem can be that the farmer used a hybrid of some sort that won't necesarily breed true.
Last season I bought a N. GA. Candy roaster at the farmer's market for $4, ate the squash (delicious) and got a pound of seed from the thing. Lost the plants to Powdery Mildew so bought two more this year and will try again.
Having said that, your seed should grow next season if you allow the squash to 'Mature'and dry the seed properly. Butternut will last all winter stored in a cool basement
so save one for a few months and then 'harvest' the seed. You'll probably do as well as you would with 'store bought seed' without the expense.
All gardens(and farms) are experiments in motion so go for it! This past spring I wanted to try my hand at Garlic and 'Seed Garlic' is upward of $20 a pound so I bought six pounds of Garlic at a Super Wal-mart for $3.89 a pound and planted it. It grew, looked different than the parent (a hybrid) but tastes like garlic and made me happy. Sold some at the Farmer's market ( made a hundred dollars that day) and have six pounds of my own 'seed garlic' to plant in a few weeks. Now that's smart gardening. Let us know how it works next season.

Buttonwillow,

The seed from store purchased squash will almost certainly be viable. Crossing is the greatest problem with such seed. Commercial producers are not trying to produce seed, so they often plant more than one variety of the same species. When they do this, the plants cross a whole lot. This is less problematic with butternut squash and their ilk, as they are all used in approximately the same manner. Also, there are fewer varieties planted commercially, which are from the c. moschata family.

It is more often a problem with acorn squash or delicata squash, as they are in the same family (c. pepo), as are almost all the summer squash and also some gourds. So some crosses end up being unusable.

Seed from Buttercup, Hubbard, Turks Turban or Red Kuri, etc. Are members of the c. maxima family. They will all interbreed freely. But with the exception of Big Max and some of the giant pumpkins, there are no squash, from this species, which are not used as winter squash. So even a cross should make a good winter squash.

But to answer your question: the seed from a store bought butternut will most likely be good and produce a delicious fruit for cooking, much like the fruit from which it came.

George
Tahlequah, OK