Low-tech or low-cost technique for quantitative estimation in enzymology
If there is a way to:

Make the replication of a common culture organism (e.g. E. coli) dependent
upon the inhibitor or substrate of interest, possibly with a restrictive media.
Express the enzyme of interest in a relatively controlled manner in such organism (e.g. well caracterized promoter in plasmid with well characterized copy number),

Then you can get good mileage out of some serial dilutions and a OD600 meter. Even better with a plate reader.
Exact protocol will depend on the quantity under consideration, but mostly will amount to substituting around in the enzyme kinetics equations until the quantity of interest is a function of the concentration, and devise a scheme to cancel out the other unknown quantities (often substrate concentration ramp + starting OD ramp + curve fitting)