Phones4U have the HTC legend on Vodafone for just £20 per month, for 300 minutes and 3000 texts. The only downside is a 24 month contract.
Delivery is free and there is £35 Quidco, so the price over 2 years is £445. Given the SIM free price of the phone is £369 and Vodafone is a good network, this seems like a good deal to me.
You are therefore paying around £3 per month for all those minutes and texts!
If you want mobile internet (in addition to wifi which is obviously free) I think the best deal is from Vodafone direct. 300 mins, ultd texts and 500mb internet for £25 per month for 24 months. There is £90 Quidco, so the total price is £510.