Synth paste should be CHEAP--that is why it is real competition to farmers. As paste, it's easy to ship, easy to process and has long shelf life.
To make it PROFITABLE, instead of raising the price (which would eliminate its primary market advantage), make it a FRACTIONAL commodity in SIZE.
By this I mean, one cargo unit would hold MULTIPLE units of synthpaste. So while an individual "packet" of synthpaste would be "dirt cheap", transports could carry so much of it that it would be a profitable commodity AND can be made a really profitable run for shippers instead of another "pretend you are making money for role play" run.
By varying the size, you can make it work for Synthfoods traders without skewing the whole Sirius market. Small storage size and non-perishability is something that WOULD make it very attractive to stations and bases.