How much does an NDB cost?
Second hand or brand new?
As avidflyer suggests the cost of antenna equipment and site are the biggest factors!
If your just replacing the transmitter and not replacing the antenna, you may be able to justify not having to flight test the hole thing again.
Two companies spring to mind that can supply them. Fernau Avionics (UK) and Southern Avionics Company (USA)
Cheers