Cost to repair 2001 nissan altima knock sensor and misfire on cylinder 4?

i have been trying to search online but can't find info. the mechanic said $900 to fix it. is that about right? the check engine light came on sun and i took it to autozone and the knock and cyl 4 misfire codes came up so i believe the garage. but the autozone printout did have other possible reasons than replacing everything like maybe it's a spark plug or something less expensive. but either way is 900 a decent price? if you know a link where i can check stuff out, lmk