Although SIPs frequently cost more to build with than minimum stick frame specification, they are generally less expensive when using comparable strength and insulation specifications. So although a 4 1/2" SIP is about 20% more expensive (on average) than 2x4 wall structural and insulation materials, it is actually equivalent to 2x6 structure and insulation.
In addition, the building process using SIPs requires more pre-planning to get the panel shop drawings perfect. This has the effect of making more of the problems get solved earlier which has a beneficial impact on the budget and the schedule. It is much less expensive to make your mistakes on paper than in the field.
Typically, for a custom spec builder, with substantial experience with a group of plans, the budget is set and exceeded by 15- 25% regularly and the schedule runs over 4 to 7 weeks- also regularly. This is generally due to last minute deal shopping for windows & doors, plumbing & electrical fixtures, appliances and down-grade adjustments made as surprises happen. Since SIP homes are substantially better planned and more issues come up earlier, it is common for SIP homes to come in on budget and on time. In addition, that on time schedule is 3 to 6 weeks faster than stick frame (scheduled) and 10 to 13 weeks faster in actuality.
So although the budget for framing is generally 15% to 20% higher (2% to 5% higher on total construction cost), SIPs have a good history of coming in on budget and are typically on schedule.