Transfer values between tabs in Numbers for iPad
There are two ways, depending on what you want to do.

If you just want to copy the current value or formula, make sure you are not in edit mode (i.e. there should be no keyboard or number pad visible). Click once on the cell you want to copy, pause so it doesn't think you are double tapping, and then tap again. A popover menu should appear. Click copy and then go to the other tab. Select the cell where you want to paste and repeat the "slow motion double tap." Then click paste.

If you want a cell to reference a cell from another tab you can do this to. Start by (quickly) double tapping the empty cell where you want the value from the other tab to appear. This should put you into edit mode. Push the equal sign button. Now navigate to the other tab and select the cell with the value you want to reference. Tap the cell once, and push the green check mark.