You shouldn't lay floor tile over osb. If you have to pad up the floor you should use a backer board or at least BC Plywood. The thinset won't bond properly to osb and the tile will fail.

Bathroom costs add up really quickly. Plumber is $800-$1200/fixture, tile probably $30-$50sq ft all in including material and substrate (depending on quality), probably a $2k allowance for electrician, $2k allowance for miscellaneous framing, say another $2k for finish carpentry including vanity depending on trim details (trim choices like crown, wainscoting, chair rail etc will increase this allowance), plus board, plaster, paint. You're also paying for design, demo, and dumpster.

Bathroom and kitchen rennos are disproportionately expensive but generally are the only interior remodels that add quantifiable value to your house in terms of future sale price.