I have two tables (Table A Projects); one with lots of high level data on multiple projects (size, location, date); and (Table B Costs) with lots of high level data on each of those project's itemised costs
Having summarised the costs per project in the second table and grouped them by Item Type, is there a way to link the summary cost into a field in the first table?
I can't see a way of adding new records to the Cost table by selecting the Project from a drop down nor a way of using Lookup to match the records by Project Name
Yes - by using conditional rollups. Make sure your tables are linked and then add a new rollup column and sum the values based on the type. Here's an example base: https://airtable.com/shrhwNZBACJRCzUe7 and a form you can use to add more costs.