How to get a sub-total next to List Custom Field

Hi,
I'm setting up registration for a trade show. I want registrants to be able to request tables for their booths. Each table is $30. I want the sub-total of this amount displayed next to their selection. So let's say they select 3 tables, I want $90 to be displayed next to this. How to accomplish?
Thanks!

Sorry Jeanne. This is quite complicated and not supported in the extension

Tuan