Ceramic floor tiles
OK, so you had peel & stick vinyl tiles and they are now gone. Was there a sheet of 1/4" plywood under them? If so, that has to go to the curb too.
Your subfloor is 3/4" plywood? How about the type and size of your joists? Their spacing? Unsupported span of the joists? Are there markings on them telling you the species and grade?
Obviously I recommend Ditra over a CBU. You think it's too expensive, if 60-80 cents a foot is going to make a difference causing a compromise, maybe you should consider something else?
Tile 4 You LLC Troy, MI
KERDI Shower Specialist ... DITRA Installs ... Product-Method suitability consultation. I have NEVER made a mistake, I thought I did once...........but I was wrong! - PRODUCTS ADVERTISED WITHIN MY POST ARE NOT ENDORSED BY ME!