Under a $100 Harris is probably the best bet. If you're talking about a bipod under $40, I've got no complaints with some of Utgs offerings. I have a UTG qd bipod that my daughter uses for her 10/22 and 15-22 and ive used it in the past to sight in a few Ar builds as well. Works just fine for light duty use.
Way back when I was into rifles, I had a Versa-Pod. Can't remember the specific model, but I thought it worked well.
MagPul is making bipods now as well. Might check those. I have no idea how good their bipods are, but usually MagPul is decent stuff for decent prices.
I just received and mounted my MagPul bipod. $100.00 on Amazon. Solid polymer with metal inner extenders, good cant and rotation. Can be bought to mount with picatinney rail, key mod or stud. I was looking at the Atlas and then Atlas clones on eBay also.
so far I like it. Good rubber feet. Mounted right now on a .22 lr rifle. It does have some play and can be front loaded for larger recoil calibers