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.
»www.usmotors.com/Our-Pro ··· cue.aspx
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.--