What's the cheapest way to do basic EMI testing?
If my time was worth nothing, or I was doing this as my own fun project, I would design and build my own rf power meter. There are schematics available for this in AARL back-issues.

If this is for work, I would go and rent the tools.

If this isn't for work, and you don't know very much about RF, and you don't have a lot of money - go make friends with amateur ham radio people in your area and butter them up.