Price is going to depend on quality. You can find some really cheap pieces that are decorative. If you love it, buy it. If you want better, you can tell the minute you walk into a shop.
Sorrento has some in expensive pieces. If you want nice, original pieces, get thee to Ravello on the Amalfi coast. There are two shops in town, both owned by the same family. They make their stuff right there and have some beautiful collectible pieces.
I never buy more than I can hand carry. I don't want to pay for shipping, and worry about something getting broken. I usually buy one or two nice things that I can't live without.
A mug can run up to $25 US for good stuff. platter will depend on the size, but you are talking $100+ and the same for a nice decorative piece $100+, depending on size.
you can find really small collectibles that have no use but to look at them for less than $10.
I bought some beautiful canisters last trip and paid $75 each (they hold two kinds of salt by my stove) I paid $65 for an olive oil bottle a few years ago.
These prices were when the dollar was about where it is now. Italy is in a big depression, so prices may have gone up, even though our dollar is about where it was 3-4 years ago.
you can also ask for a discount if you buy more than one piece, and they will offer you the VAT refund on your purchase if it's over 100Euro (this could be up to 20%)
As far as Rome goes, it's not a small town, no small pottery shops that I saw, just cheesy plastic Forum and Vatican "collectibles".
Save your pottery/ceramica shopping for small towns.
You might also check into going to Orvieto for a day, they have a really nice shop (and some not so nice!) I bought the most beautiful "Biscotti" jar and matching bowl last year. $125 for the jar and $35 for the bowl. Not cheap, but I use both daily!