I don't think it will be cheaper and I haven't seen any products that coat existing countertops.

You most certainly can have a concrete overlay put over your existing tile. As far as being functional, I would say that it is very functional as in you will be able to have all the existing cabinets and appliances being in the original layout. As far as "cheaper" there are too many variables in play, as to whether the original tile is in good enough condition to be able to accept the overlay or if extensive prep would be involved, what type of design or colorization you may be looking for, and what other products you would be comparing the cost of the overlay against. Look for a quality concrete artisan in your area and contact them to come out to your residence for a inspection of your counters and to go over the options they have to offer.