i have a car computer that i always found a pain to get online (have to use my phone, but then i need my phone to talk on, ugh). so i had to look for the same thing as you- very cheap mobile internet.
boost mobile was the clear winner. its not as sweet now though, because they bumped the price up to 50 cents a day... but still thats only about $15 per month and you only get charged for the days you actually use it.
setup can be a little costly ($30 to activate a sprint phone using sprint2boost.com). you need a sprint winmo6 phone, as far as i know that is the best way because there are numerous apps for tethering to make it easy. winmowifirouter is tops in that regard. so if you have to buy the phone and winmowifirouter, plus the sprint2boost activation, you are looking at $100 upfront cost. however, it is possible to call cs and get lucky - they might activate a sprint phone for you for free. and you might have an old winmo phone laying around... plus there is free tethering software if you dont mind a little less convenience. what im saying is you might not have to spend any upfront cost at all
but like i said, you only get charged the 50 cents for that day when you first connect. so if you go a day or two without using the boost internet, you pay nothing. downside is, even if you just connect for 1 second and only use 1kb of data, it doesnt matter. you will get charged the 50 cents for that day up until midnight (even if you start it at 11:59pm, you only get a minute then before you get charged another 50 cents for the new day at midnight).
thing is, i like it this way because its simple and the data limit is non existent. just pay the 50 cents, and abuse the connection all you want for that day. (some say boost does have the 2gb limit, but it is a sprint service so i doubt its a hard cap).
overall, i love it. i just figure at most i can only get charged is $11 for a month (35 cents a day for me) of mobile unlimited internet. and most months its more like $8 because i dont drive every day.