We always chuck any old compost from pots, back onto the garden or onto a new 'raised' bed (it's a bit higher than the rest, as it's the old compost heap), and use it from there for everything - including tomatoes as long as it's at the bottom of the 10" pot. (Isn't that OK Mum)?
After all, it's only a medium like soil, and all it needs is some freshener-feed, and a good dig over to get the old roots and dross out!
I suppose we use about a dozen 70 litre bags of new stuff a year, for kicking things off and salad stuff, but the old compost gets plonked just about everywhere!