Gps is so expensive,
Is there not software out their that updates a 'road' tomtom' etc into
aviation gps?
Yes! Better still it is updated at least monthly and costs less than the TomTom road updates.
Unfortunately it does not run on TomTom GPSs but it does run on most other makes of car GPS. TomTom use a somewhat "strange" platform.
Have a look at PocketFms.
I reckon PocketFms should be more widely recognised. While I dont use it in the aircraft (although I did for many years) it is also excellent to run on your PC for planning and well worth the annual subscription.
(I have no commercial or other interest in PocketFms).