I had two bags of the cherries which I defrosted and simmered in a little water. I asked my friend if she wanted whole cherries or cut up cherries and, in the end, mashed them a bit with a potato masher which worked great.
Then I measured what I had and added sugar in the proportion pint to pound - in other words for every pint of fruit I added 1 lb sugar.
Now, cherries are pretty low in pectin so I used jam sugar and also added a good squirt of bottled lemon juice.
I added the sugar and lemon juice to the cherry mix, gave it a good stir and left it, covered, overnight. I've discovered that when I do that, less boiling is needed and setting point comes very quickly.
So the next day, when the sugar had all dissolved, I boiled it up, checked for set (got it first time) and bottled it. Today I will be off for coffee with my friend, with several jars of jam and a jar of marmalade. I love bearing gifts!
As for cost, well, no, not cheap, certainly not frugal! The cherries were £4 (although they are sometimes on special offer) and the jam sugar is more expensive than the ordinary sugar at £2.00. But, when you can't normally find that jam, it is worth it and it's not more expensive than comparative quality bought jam (I checked My Supermarket).
And making jam is a very satisfying experience!