Tip: Don't look at the cost on the top left of the card in the lab, instead look at the top right of the screen. I have found that the cost on the top left of the card often has the wrong # of 0's
You should add something that says YOU CAN UPGRADE AT THE TOWN or something so that you can know when to go to the town instead of just going at random times to see.
A good strategy if you're stuck is to capture all of the empty cells first, then start attacking the less powerful enemy cells, working your way up to ganging up on the more powerful cells.