Let's see..
Smarty unlimited data on Three network at £18 per month.
TPLINK 4G router for £73.49.
Over 12 months that is £289.49 or £24.12 per month.
I am not recommending either but that took me about 2 minutes to find. Other routers I am sure would be available for a similar price that may be better - you would probably be best doing some research into 4G routers. Smarty is the only 4G I am aware of that would come in that budget including cost for a router but if you are happy to do some money towards a router for initial outlay then getting under £25 per month with other 4G providers may be possible.
EDIT : Corrected network
Thanks that a great help (wish I could find such detail in under 2 minutes, I bow to your search skills)
The only other thing is will the 4G reception be any good where i live? Now that can be found with trial and error I guess. I hear a lot about the THREE network being poor in London but good elsewhere.
I have heard of smarty thanks.
People I know say EE is the best mobile provider.