The last rectangular tubing I bought was $4.89/ft for 2x4 x 11 ga. Steel prices have increased a bit since then, but 2x3 should be slightly less than 2x4. So, I'd expect that somewhere between $4.50/ft and $5.00 should be in the ballpark.
I bought 2x3 1/8 wall about two months ago and paid $4.80/ft. You might be able to get it for less in a bigger city. I'm out in the boonies of northern WI and have to buy my steel from a pier builder - not a steel supplier.
Also, the 2x4 tubing I bought was $6.10 a foot - so quite an increase there. 1x2 was $3.10 and 1x1 was $2.10.