We have just been looking into what we will do when we sell our current home. Looked at traditional construction, monolithic domes and modular homes. Of the three types, for us, the order of preference was:
1) modular - better value, quicker construction times, better construction control and better materials
2) monolithic dome - better control, quick construction and better disaster protection.
3) traditional construction - most expensive, subject to weather hold ups, often shoddy work etc.
Whilst the above is not absolute, it does give a good comparison of each type of build. The modular come in around 40% cheaper in central Canada, over the cost of site built. Depending on the company, turnkey construction methods available where the company looks after the foundations construction, placement, installation, hookups etc. - and they are still cheaper than the site built equivalent home. For specialist construction and where the owner can contribute to the building, onsite is probably the better route.
For us, when the time comes (after selling current home), we will go the modular route - quicker, cheaper etc. In the meantime we can live in a rented apartment for a year and get properly organized.
Of course everyone's needs are different, but for us the modular homes offer the better solution.