I carry O2, Three, T-Mobile and Vodafone cards for my unlocked MiFi.
O2 came with a dongle - I don't use it. I don't use the Three SIM either that came with the MiFi.
T-Mobile do £2/day, £4/three days, £7/week access and Vodafone do 1GB/£15 no expiry (but you can get these SIMs pre-loaded on eBay for £10).
You can't do any of those offers with a stock MiFi that is locked to three.
As for £10/1GB on Three - you know that the 1GB expires after 30 days? That means you will be spending around £120/year on a rather limited MiFi data tariff.
I'm not sure the Three data allowance has an expiry. I know the initial data allowance that you get on the PAYG + 3GB pack has an expiry of 3 months, but I can't find any info on the top-ups have a 30 day expiry.
I don't quite understand how unlocking works - if I had a Three MiFi and unlocked it, and used a SIM from somewhere else, do I still use the Three 3G network? But now I'm not paying Three to use it, I'm paying the other company? Sorry, I've never used 3G before so am not sure how it works.
Edit: Actually, found it on the Three website - "There is no expiry date for top-ups."