Yes, it is to do with number of channels available for data. Mobiles have some channels for data and some for voice. USB dongles only need data channels.
Contracts for USB dongles range from abouy £10 to £25 per month.
You need to look carefully about how much data each one allows.
Plusnet Plusnet Pro 21CN Netgear DGND3300v2 router
Windows XP Pro SP3