In my experience, "lead deposits" are basically an issue caused by running excessively rich mixture, usually on the ground, and show up as spark plug fouling with engine roughness and reduced RPMs. The fix is just to vaporize the deposits by running the engine at leaner and at a high power setting.

I understand that you can also end up with exhaust valve deposits, but that's not something I've seen personally. Aviation engines (especially those used for training, which is a large proportion of the piston fleet) have incredibly short inspection intervals in any case.

This is unlikely to be a major factor for aircraft engines, which need to be serviced regularly anyway (long before lead deposits would force it.)

According to http://www.100ll.com/ - about $6.00 to $6.70 at retail.

Looking at my own local airports ( e.g. http://www.100ll.com/showfbo.php?HashID=d0f7e0afbdcf9b9a7cde... ) the price goes higher to often over $7.00/gal, though that's for "full service" (I don't know what that means, though).

You don't want to know. Somewhere between $5-10/gallon. Normal times, $3.50-7/gallon.