Sorry, but I think it could well depend on whether the line is still operational (and in particular, whether there's wire from outside to the BT "Master Socket").
From time to time there are deals from BT Retail where they will do free installation but in return you have to pay their monthly fee (around 15 quid a month, whereas some others charge under 9 pounds) and to make a number of calls each month (but you can do so on the weekend when calls are free!)
After 18 months you'd have paid them (at 15 quid/month) 270 pounds just for the phone line (and free weekend calls). At present there's a deal on where they will provide a line (normal cost 130 pounds) if you take a broadband and calls package.
The broadband is 9 pounds for 3 months and then 18 pounds a month, and you'd still have to pay line rental of 15.45 a month. They will let you pay 129 pounds for 12 months rental in advance (making the monthly equivalend 10.75 [but it is non-refundable payment, to stop people switching to TalkTalk or Sky]
I would have mentioned PlusNet as they had a deal on, but they have a 49.99 line connection fee (and while regular line rental is 13.99, it only comes down to 10.49 if you pay for the full year in advance).
Unfortunately, it really depends what you can afford as to what's the best option, and also to some extent, what you want to use the internet for - at the cheapest prices there may be limits on how much you can download.