Very high (99%) insertion cost in estimated execution plan

I have four tables with 500,000 to 4,000,000 rows. The script joins them and insert into a temporary table with some where clause. It is taking long. The execution plan shows 99% cost only for the insert whereas there are multiple scans in the plan. Snapshot of the related part of the execution plan is shown in the image. The drive is solid state. How do I interpret the plan. SQL Server execution plan