Amazon offers Prime members the Nordic Ware 10.5" Microwave Spatter Cover, model no. 65000, for $1.79 with free shipping. (Target offers it for the same via pickup. Non-members can spend $49 to qualify for free shipping; otherwise, shipping costs $6.22.) That's the lowest total price we could find by $6. It fits most standard dinner plates.

Update: It's now out of stock at Amazon but is still available at Target.