SparkFun posted a great primer on FCC approval for hobbyists. Agency approvals are sometimes forgotten by entrepreneurs and inventors with horrible results. Here are some high points:
Even devices which do not use radio transmitters?
Yes, because in the right (or wrong) circumstances, any device which contains a relatively high speed clock (above the low kHz range) stands to possibly produce unwanted interference to other local devices. Devices which do not use radio transmitters are referred to as “unintentional radiators”, and the testing bar is lower for them than it is for intentional radiators. Thus, all devices which cannot be exempted must obtain FCC authorization prior to being marketed in the US.
In other words many devices that contain no radio or wireless functionality will require approval as an unintentional radiator. Does your device contain a processor? Then the odds are very high that it will need approval.
How much does it cost to obtain authorization under the FCC rules?
That depends on your device. Devices which require Verification or a Declaration of Conformity- which is to say, unintentional radiators- can be tested for about $1000. There is usually an additional fee of around $500 for a report which may or may not be needed. For intentional radiators, the Certification cost is more like $10,000-$12,000, unless an approved module is used. It may be that you will fail, of course, which will require retesting.