I started with a used 104hp powered Make Noise skiff for about $200 plus tax. If you want something versatile, Mutable Instruments' Plaits is a good place to start looking, and you might be able to find a used one for about $200... and there's your budget.
But what about all of the other stuff to actually make it work? Cables, filters, LFOs, a sequencer, envelopes, effects, blind panels so you don't accidentally touch a cable to a power header and short everything out...
If you really think that getting into modular is the right direction for you, spend your $400 on a decent powered case (TipTop Mantis or small Doepfer for example), and then save up again until you can afford your first module. Then save up again for your next module. Repeat, repeat, repeat. I don't know your financial situation, but it may take a considerable amount of time until you have a useable system. Whatever you do, don't go into a large amount of debt.