Shims are £3 each secondhand - I'd expect new ones to be a good fiver or so.
Changing the shims is a WHOLE lot more work than checking them. To check them, you only have to get the valve cover off. To change them, you have to lock the engine at TDC, take the camchain sprocket off, install the camshaft tensioner bolts, remove all camshaft bearing bolts, remove the camshaft bearings, then lift out whichever camshaft you need to get to. Then you can change the shim, and do it all back up again. To the correct torque. When I do it, it's around an extra hour's work for me, over just checking them.
Steve