I bought a netbook for 12000 thb, but was given an edge modem dongal for mobile internet access for free (had to ask though). AIS have good deals on internet time, buy a sim card (50thb) put it into a mobile phone then buy credit. Next you need to assisn that credit to internet time.
To convert, dial *138, then when prompted, 1 for english, then select 2 for mobile net package, then 1 for monthly package. Now select the package you want. The package is autorenewedafter 1 month, but beware....if you run out of internet time before the end of month, you will be charged 0.75 per minute again.
From memory I bought 250 hours for 300thb plus 7% tax, unlimited is another option for 535thb (incl tax) The credit is taken off your sim card immediately.
If you run out of minutes before the end of the month, then cancel the contract and start a new one, otherwise any additional time purchased may expire at the end of your month, i don't know