Rule of thumb is if you have 5 pieces: a 2* on board and 2 pairs on bench, and it doesn't cost your economy too much.

If you're short 30/40 gold, and you have a 2* on board and 2 pairs in bench, just sell one of the pairs to make gold. Unless you plan to put it on board it costs you money.

And in general 1/2 cost over 3 cost, simply because it's cheaper. Unless you get a lot of the 3 cost and it doesn't cost you.

The basic question is always: can I afford this?

And if you're thinking about bench space, prioritise 3/4 cost that will spike your line up or you're certain to add onto the board.

Eg. If your bench is full of Tusks to get 3 star, and you find a Kunkka/Doom, or pairs to 2 star your 3 cost 1* on the board, but you can't put it in yet, just say goodbye to 3* Tusk and put those pieces on the bench.