Are you sure it is only floor???..again, if you are just changing the floor tiles and only doing water proofing for floor area, you are likely to have leakage issue..waterproofing at least better to do for 5 feet wall height..

If seeing from your 28sqft floor size, i assume your bathroom size will be 7ft lgth x 4 ft width x 9 ft wall height..

Total area wall + floor will be approx 226 sqft. My contractor charge me RM7/sqft to hack off,to screed, waterproofing and to relay tiles up to 5 ft height per my request..balance 4 ft wall height is calculated at RM5/sqft to hack off,to screed and to relay tiles..tiles excluded

So if 226 sqft=RM7 x 226 = RM1582..I would say this is quite close and more logic..for waterproofing material(sika top) it will cost another RM 115~120

In my case, I actually count to actual sq ft..to minus the door area of 18sqft and even the casement window another 8sqft