Is the 250k tons what you can see, or what you think is there based on what you can see? Keep in mind that fortification reduces the signature of ground units, so if you can see 250k tons, and it's fully fortified, it's probably more like 1M tons of ground units.
If you can't afford the ~3M tons of invasion force you'd need to defeat that, then your only option is to bombard. If you can't afford to bombard the planet into uselessness, then you'll have to back off and find resources elsewhere.