Considering the resale value isn't much better at rich industrial worlds than poor agricultural worlds, please reduce this to only ~30 tons ...and/or make it more variable between 10 and 60 tons.
As it is, even 8 asteroids (such as found around 1 rock hermit asteroid) is often more than enough to "flood" the market and leave you with minerals you can't sell.
And with a Cobra Mark III, you'll even need a few trips to do so.Game-balance wise, being an asteroid miner is risky enough, what with the time involved doing so...and even as I've found running into asteroids and now even boulders (in v1.74 trunk builds) at 25-50% speed can be fatal. If someone's searching around a system for asteroids, they'll likely kill lots of real and ingame time just trying to find asteroids. (OXP add-ons aside that can make this easier.)