This is the most commonly ask question from any client who wish to start an online hotel booking website. But unfortunately it is relay difficult to even give a ballpark cost unless they can provide bit more details about their idea. Their are four main types, based on how hotel rates and availability handled when building a hotel booking website.
Net Rate Based
Hotel Net Rates and availability obtain from a hotel wholesale supplier , DMC (Destination management Company) or from hotel directly through a XML connectivity. You have full control over margin and markup.
Same manner as above, you can receive rates and availability through various sources, but you will not have control over prices. You will get commission based on number of bookings, booking value, site clicks or impressions. Commissions and commercial structures are varied from one supplier to the other.
Direct Contract/Bed Bank
This means you should have direct arrangements with individual hotels or hotel chains to get their travel agency rates.
Fourth type is building a hotel booking website combining all four above types.