If the BFR (the rocket behind the starship) is successful then yes it could mean the ability to launch very large telescopes in to space. The scientific community would be very exited about this possibility. However, this doesn't necessarily make it very low cost. One launch of the BFR would still likely be much more than an a Falcon Heavy launch.
> if you are building a radio telescope in space, could you just use a thin foil that folds out like origami for the reflector?
Yes! This technology already exists and it is really pretty amazing to see in action. Right now most of them are used on communications satellites or for synthetic aperture radar satellites. See the videos below:
Animation of the radar antenna on SMAP:
https://smap.jpl.nasa.gov/resources/83/smap-antenna-deployme...
Actual video of a large communcations antenna (12m diameter) being deployed. Skip ahead to ~2:15 for the actual unfurling.
https://www.youtube.com/watch?v=_mFnNDzxKFk&feature=emb_titl...