I haven't priced wireless sensors in a while, but last time I did, the Elk 2-way sensors, like the 6021 mini window sensor, were about 2x the price of an equivalent Honeywell or GE/Interlogix sensor.
I liked the concept of the Elk two-way sensors when they first came out. But I've been disappointed with how slow Elk has been to grow the product line, and that the price has remained so high.
These days, if I was configuring a new wireless system, I would go with Honeywell 5800 series sensors rather than GE/Interlogix, since GE/Interlogix has abandoned the business. GE/Interlogix sensors are still available, and there are a few other manufacturers that make a limited subset of compatible sensors, but why paint yourself into a corner?