Agree with roadwalker - I did some checks on the Three site...
A quick comparison: Nokia Lumia 520 on contract £16 / month gets you 300 minutes, 5000 texts and 250MB (£16 x 24 = £384 )
Buy it PAYG in Three (brand new) at £99
You were after 1 GB of data.
Essential Internet SIM 600 (12 months) gives you too many minutes but the amount of data you were after...
600 minutes, 5000 texts, 1 GB (£9.90 x 24 + £99 = £336.60 )
Essential Internet SIM 200 (12 months) gives you 200 minutes but less data than you were after...
200 minutes, 5000 texts, 500 MB (£6.90 x 24 + £99 = £264.60 )
Both are cheaper than the contract "deal" and although no direct comparison with minutes/data being different in every case, it looks like you could save £120 buying the phone up front.
It's worth explaining what you are wanting (and why) if you go in a Three Store so they will "waive" the mandatory top-up that they normally expect when buying a PAYG mobile.
Clearly there may be alternative deals.