How to get a cheap VFD for 230V to 400V?

Even with a constant speed you will have to start and stop the motor. If you just have an IGBT power block producing a fixed frequency you will have to deal with the high inrush current, 3-5 times FLA. The are several companies that make chassis or open style VFDs that can be around 30% less than the standard IP20 versions with all the fancy stuff.

I have heard of some issues with high failure rates with these variable frequency drives but I don't know what the cause is. You will probably have to buy 50+ at a time to get a good price. Make sure you KNOW the requirements for your application before you order 50 VFDs. There are inexpensive drives that are very noisy with high voltage floating on logic I/O that can cost more than the drive to isolate. Make sure the drive can handle the range of line voltage and hard starting requirements.

Most 230 volt VFDs will accept single or three phase up to 3 hp without oversizing.
As most have indicated, a 1 Phase VFD will be the most economical solution, but will, only give you 3 Phase 240V output. They cannot give you more output volts than input volts. In marine industry (small trawler operators with 1 ph Generators) there is a special transformer manufactured which will give you 3ph 400 volts, but expensive and very limited market.