What are the costs versus benefits of takeoff/landing on grass versus pavement in a light aircraft?
Pretty simple.  Even a nicely rolled grass runway is nowhere as smooth as asphalt.  

Tires on grass seem to last forever, but it wears out everything else in the gear out faster.  Oleos get worked more; they get dirtier, so it's much harder on the scrapers and seals (dust (silica particles) + hydraulic fluid on the chrome = wonderful lapping compound), etc and can be expected to start to leak sooner. Scissors linkage bushings  and wheel bearings wear faster.  Fairings and bits that flex with the gear will crack sooner.  All that sort of thing.  Impossible to quantify, although tires are cheaper than oleo overhauls.

I operate from a field with grass and pavement (actually macadam - gravel/tar) side by side and I almost always use the pavement, because on smooth grass I can feel the oleos getting worked hard and they just get a lot dirtier.

On the other hand - if it's an option, I always do my engine run up on grass, and I usually start the takeoff roll (an option where I am) on the grass about 50 ft short of the threshold.  These are propeller protection measures.  Even nice clean looking pavement will have lots of, at minimum, very small pretty much invisible stones, which are murder on your propeller when stationary or when rolling at low speed/high power.  

Props make great vacuum cleaners when close to the ground and pavement is like vacuuming a tile floor while grass is like vacuuming carpet without the beater attachment doohickey.