As above, scour ebay for something 2nd hand and interesting.
My Plug cost me £100 off the bearbones forum (good bits were the frame, finishing kit, BB and cranks, everything else was knackered. Think it came with a decent pair of tyres as spares.
I then spent about £300 shopping around for bargains, building new wheels around a dynamo hub and DT rims, dynamo lights, new (better) v-brakes, mudguards, new chain, sprockets and chainring.
It’s nice enough to get the occasional comment from cyclists (usually that it’s just a very practical build!) and rides incredibly nicely. But is dull enough not to be a thief magnet.