There would inevitably be time difference in designing a website from scratch vs. taking an predesigned theme from somewhere. So, the former takes more time as custom web design projects need high collaborations like presentation, feedback, revisions etc. these loops of interactions take place every time on the basis of the team structure and it will take time to accomplish. So as the old adage goes, time is money, which means the projects made from scratch cost more than the alternative.
How to determine the time the project will take? Firstly, you need to have the specifics or the amount of work that has to be accomplished. This is defined in the discovery process itself. Secondly, you need to have the team structure in place and check what the revision loop looks like. A bigger client team that consists of many stakeholders will have more extended design process than a smaller and more agile company. A team that needs many revisions will always invest more time during design than the one being approved quickly.
And due to this, clients need to understand that website designing singapore projects will take longer and will of course cost more than the alternative option of going with licensing a layout or interface. Hence, this is why web design proposals tend to vary in cost. Many designers will design a layout while the others may just use a theme. The confusion is valid for clients when they cannot tell who is going to work on what.