Any VFD I know of would be a quite expensive but an upgrade to an ECM blower motor may meet your requirements while not blowing your budget....
RESCUE EcoTech® - The High Efficiency ECM upgrade for PSC blower motors
The RESCUE EcoTech motor is an ECM replacement for a PSC blower. Exclusive speed-control technology provides the efficiency of an ECM. Installation is a snap. RESCUE EcoTech drops into existing permanent split capacitor (PSC) direct drive blower applications, without complex wiring modifications or time-consuming changes to system controls.
Electronically Controlled Motor (ECM) technology gives RESCUE EcoTech motors up to 82% efficiency, saving 25% or more of the watts in A/C or heating mode and 75% of the watts in circulation mode. Updated designs are now dual voltage (115/230V) and multi-horsepower, so one size, or at most two, fits all your needs.
These motors offer soft starting as well as other speed options. The local HVAC supply house has the dual voltage, multi-HP 5552ET direct-drive blower motor priced at $180 so you can probably do better on the 'net somewhere.--