I have a small single family house with 100 amp service. By my calculations, 100 amps is fine but the panel only has 8 circuits and I am adding a number of circuits with a bathroom/kitchen remodel. I am thinking that adding a subpanel for my kitchen will be cheaper than replacing my main panel with a newer model with more "slots".

The panels at my local building center are 100A minimum. Is there any harm having a 100A sub on a 100A main if the total usage doesn't exceed 100A? Or should I install the 100A subpanel with a 40-50A main switch?

There is no problem with what you propose.