I am a network administrator for a company with about 1,000 employees. For that short of a run, I would absolutely run a 2nd conduit and run wet rated cat5e. You cannot run low voltage and hi voltage in the same conduit, that is a code violation for safety reasons. 30ft run is no big deal, you could do that on a sat easily. That'll work to 1 gigabit or 1000 mbps
If that just is not an option, I would recommend the power line adapters as others have mentioned. you will be severely limited in bandwidth though.
a 3rd option is to use a wireless range extender to amplify your house's wifi, but again this is a lower speed solution.
Optimally you will put in a cheap 1/2 or 3/4 conduit, run cat5e, and take a wireless router to the garage, put it in "bridging" mode, and that will extend your wireless network out to the garage through the Ethernet cable, as well as give you 4 wired ports from the built in switch. You can make the SSID different than the one in the house so that your devices in the house don't try to roam onto the garage network. Secure them both with the same WEP or WPA2 key
Be forewarned though, electronics don't like extreme heat or cold and might flake out. this will affect any of the 3 solutions though.
Happy Networking