What kind of cartridge are you paying $100 each for? How big is it (diameter/length/type) and what micron rating? The Harmsco PP-BB-20-1 http://www.harmsco.com/uploads/pdf/h...at_catalog.pdf has an area 4 times the area of a 2.5" x 10" cartridge and has a list price of $75.
The usual practice is to put something like a string-wound filter before a very fine cartridge. I install systems that use a 1-micron absolute final filter with the best string-wound filter that is available. The string-wound filter collects most of the dirt and is inexpensive (about $3 for a 10" long 2.5" diameter cartridge).
A strainer of any kind will not remove enough to significantly improve the life of a fine cartridge filter.
When cartridge expense becomes significant ($100 per month for a household is very expensive) it is usually more economical to divide the flow by doubling the number of cartridges.