I'd save the money for something more worthwhile... like petrol.
The cracks in mine had been getting worse for years, but each time I had the carbs off and checked the rubbers the cracks were nowhere near full depth. Earlier this year I decided they must have finally penetrated all the way through, water placed on the cracks looked as if it was getting pulled and pushed by the pressure fluctuations. So I bought some replacements... when I got round to taking the carbs off I found the interior of the rubbers was still flawless. I guess the water test was just detecting engine vibrations.
If you really want to change them for cosmetic reasons then it's only the outer two that normally exhibit cracks so you could go halves on a set with someone in the same situation.