What is the upcharge for plywood?

I just did Kraftmaid cherry cabinets and selected plywood for the end cabinets only. The exposed sides look better that way. For all plywood, I think it was about 15% additional, so this was a compromise where it mattered. This is what the designer suggested and I think that makes sense.

$1500

I had particle board cabinets for 25 years and they were fine. We had a few issues with the pins that hold the shelves in place about 15 years in, but were able to resolve them by adjusting the shelf to a different height. There do seem to be fewer design choices when you go with particle board, but if you find what you like there that shouldn't be a problem.
With all that said, if the difference is $1500, I would go with plywood.

Plywood would be the more 'durable' option compared to particle board.