Here is Washington State it is now BLACKBERRY TIME! Most of the year I hate them, trying to kill back the vines (this year the Himalayan blackberries went nuts, probably some vines grow a foot every night--of our 2 acres, over a half acre is no inaccessible because of them). HOWEVER, right now the berries are ripe! So if you can get the berries for free, especially in large quantities, perhaps it is more cost effective. I say this because the high-quality 'boutique' jams you see in the store for $4-$6 / small jar can be duplicated with really good freshly picked fruit. Overall expense may exceed buying it in the store, but there is nothing as good as jam made from truly ripe good quality fruit.
I grew up helping my grandmother make the hot-water bath canned jam. Wonderful.
edit: I should add, you might be able to get the overall cost below what you would pay in a store if you can create a large enough batch, especially if you can get some supplies through (ahem) C****sList for cheap.
one more edit: someone mentioned using these as gifts, which can be a truly wonderful thing. If you spend $5 at a store for a trinket, and give as "the" Christmas gift, it would probably be pretty cheesy (and received as same). Spend $4 and some labor of love making a half pint of personal label designer jam--with no label, frequently--and your loved ones will be thrilled. It especially goes a long way for giving at extended family parties, where you are expected to bring something for lots of people you rarely see.