I think the point to make clear here is that TKS is actually anti-icing, not de-icing equipment - that is, it prevents the formation of ice and does not get rid of it if it has already formed (as boots do).
There is a world of a difference between certifying an aircraft for flight where possible icing is forecast (and your SOP is to get out of it quickly) and for flight through known icing. I was involved with the certification of a certain military aircraft where this point had to be well understood, as otherwise the aircraft would have been prevented from flying if there was any icing forecast, even just a thin layer. In the trials it took ages to actually get some ice on the aircraft as most forecast icing wasn't! But when we did get some, it was horrendous.
As a PPL, I would not want to fly in known actual icing conditions, regardless of the kit ...
Going back to the original post, the answer is that the difference is a lot of money!