As I don't know your dates its hard to comment.

There is a Premier Inn between on the Euston Road between Euston and St Pancras. The Belgrove hotel is often quote cheap too with rooms around £70 per night on a week day.

Further out, I would pick anywhere on the Picadilly or northern lines going north as I know these areas . If you stay out near Manor House station there are few hotels on the Seven Sisters road which overlook Finsbury park. If you are really brave you can get some good deal in Harringay too.

The Britannia hotel near Chalk Farm (7mins from St Pancras) on the northern line should also have rooms well within you price range too. I used walk from Chalk Farm to St Pancras when I worked in Paris (takes around 35mins even with a pull along), the no 168 bus to Aldenham street is 10mins and will put you within 5/6 min walk of St Pancras.

Like I a say, I am biased towards North London as I used to live there less than 6 weeks ago . I am sure there will be better quality replies.