Search around and compare numbers, that'll be your best bet. I'm not up to speed on the sub-$500 stuff, but as a general rule, the diameter of the tube does not effect your field of view. It's the diameter of the objective lens (the one facing the target) that matters. Other things like the way the scope is optically engineered will also affect the FOV. The main advantage of a larger tube diameter is that it facilitates a larger adjustment range for drop and windage, because the mechanism has more room to move.
For example, the Nightforce NX8 1-8x24 has a 30mm tube and the Vortex 1-10x24 has a 34mm tube. Their FOV numbers are pretty close (less than 10% difference) as they both have a 24mm objective lens, but the Vortex (34mm tube) has a 120 MOA elevation range and the Nightforce (30mm tube) has 100 MOA.