There are several different brands of glow plugs available today. Prices have been going up the past several years. Based on the engines and fuel you are using, you should be able to find the glow plugs that work best for you.
Rossi R4 glow plug for low nitro fuel and/or small engines, 5 plugs for $15.00
Rossi R6 glow plug for high nitro and large engines, 5 plugs for $20.00
Rossi R8 glow plug for high nitro and the big engines, 5 plugs for $25.00
Al Hobbs