I got a price from a local company DC, was quoted £36 for a 7.5 mtr length of 50 x 50 3mm box. Downside was that they would only sell me a full length which I didn't really see being a problem with what I would need. Company is called GM Steel but not sure if they would ship or what shipping would be,
Point is that if you have someone local it is sometimes a good idea to pop in & speak to them, had other local suppliers for various things who have offered decent prices when you went to see them, especially if they knew you had been looking on the net. Face to face is always better than phoning from my own experiences.