[GrimJosef]

You can pick up something like an 807 for close to peanuts and in a conventional audio amp they might well be a better bet. Transformers and chassis can be the most expensive parts at the cheap end of the market. In the end cost can come down to what bits and pieces you already have available.

EDIT: In fact in the case of an 807 it might be trickier to find the socket than the valve !

Cheers,

GJ