I believe when doing reno, the cost will be mainly devided into 2 parts :

Labour
Materials


Labour is relatively easy to calculate, but the materials price usually vary. Not all contractors gives good materials, as the good ones vary a LOT. eg., 2x2 tiles price can be RM10 pp or RM30 pp.

I would recommend that you specify the materials (or better still, buy the materials yourself and let them do the labour).