Is there an economical way to measure rf voltages below 1 mV?

If it's more a matter of relative readings you want (to peak circuits for example) you might make use of a portable (or other) HF general coverage receiver.....using either it's own S-meter (or adding one or test jacks to use an existing meter you probably have (or scope). Be wary of measuring just audio output though unless you're comfortable with how any AGC system of the receiver will affect your readings (although even this method can be useful in some cases).

I don't know what your requirement for being economical means but this "type" of idea isn't the worst way to measure low level HF (or other) voltages.

In days past especially, it was not uncommon for radio shops to have a receiver that was very much tapped into so that signals could be run through them for substitution and/or measurement purposes.

Not knowing your experience, I'll leave it at this but will be glad to talk up ideas if you'd like.

Gord - VE1AJF