I don’t have a exact cost because I also cut out the picture mat and used some scrap wood to extend the picture frame on the back to enclose the raspberry pi.
They make little 7-10" monitors that you could use to display whatever you want.
Just the 7 inch screen for the raspberry pi.
https://www.amazon.com/gp/product/B07VNX4ZWY/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1