How much extra weight is added by strengthening a piston-prop fighter for carrier landings?
The British adopted the Spitfire and Hurricane for carrier use.
Using the Spitfire as an example, here's data from Wikipedia.

Spitfire Mk.VB is 5065lbs clean.

Seafire Mk.III is 6204lbs clean.

They're not exactly equivalent otherwise, but close enough. 
So the Seafire is roughly 1140lbs heavier than the land based equivalent, or over 20%.

Of course this will vary wildly for different variants compared, but it's a ballpark figure