Like you, I'm considering trying a knock off. Birks seem to be the only shoe that work for me and they are super expensive. This fall I'm going to be purchasing their 'Ontario' shoe...it's the first Birkenstock shoe designed primarily for folks who do 'serious walking and light hiking'. As one who camps and stuff, I need something with more coverage and water resistance than my Birk Oder sandals.
Best to you.