Hmm, you'd need to :
1. Somehow trigger a script whenever a new table gets created to create the new field in "Combined Price List" for the new month
2. Link each of the records in the monthly price list to the record in "Combined Price List"
3. Update the new field with the values from the monthly price list
4. Whenever the monthly price list is updated, it should update as well
I think it's all doable the first one (triggering the script when a new table gets created), but you could just do that manually I suppose. It'd be a lot of work though I think
If I were you I'd just:
1. Create the 12 price list tables manually
2. Link them all to "Combined Price List"
3. In "Combined Price List", create one lookup field per linked monthly price list table
Not sure how the records get created in the monthly price list tables, but if they come in as a batch it'd be simple enough to just copy the entire column and paste it into the linked field to link everything up