Yup, that's gonna be expensive. I just ran 30A about 110 feet and I used 8 gauge, which according to a few online wire calculators, was right on the edge of the envelope. I'd advise you to find a reputable (looking) online calculator and go from there.
Is this a new service, or a feeder off of an existing panel? Is this for a dwelling/shop, or why do you need 100amp service?
Direct burial aluminum is probably cheaper, but I prefer copper in conduit. Regardless, I suggest you throw an extra empty conduit in the trench, you can later fish something else through. Also consider if you need water pipes while the trench is open.
Note that if you upsize the conductors for voltage drop you have to upside the ground as well.
I would always run underground wire in conduit, even if not required by code. Between the extra protection and the possibility of replacing easily if ever needed, and the relatively low cost of PVC conduit, I consider it a no-brainer.
Slime does not pay. Always keep your tiny ad dry.
poor man's poll: would you support a kickstarter for a SKIP book?