You're not making a good comparison.
The Dell 20" ($251)/22" ($296) and the Apple 20" ($699) all have the same resolution.
The Dell 24" ($674) and the Apple 23" ($999)/24" (iMac) all have the same resolution.
As you can see the Dells are significantly cheaper in every case, and have more features: adjustable stands, VESA mounting, more video input options, flash card readers, etc (features vary by model).