There are 2 types of costs - internal & external. Since you already have a management system in place the initial costs will be lower than starting from scratch. But bear in mind that ISO14001 is a lot more prescriptive & you will need to correct deficincies, not just re-write procedures.
Internal include time from your people. Typically it takes 3 to 6 mos. & you should consider at least one person "dedicated", about 20% of the time for Dept Heads, a few days for management and a week or so for other line personnel (production/maintenance staff). External costs would include audit & registration process & you likely have a good handle on that from ISO-9001. If you use a consultant reduce the internal costs by at least 1/2 & add consulting costs to external.
If you decide, based on objective review of your env'l aspects / impacts that a change in processes is needed, then the cost of making these changes should be added.
There are lots of $ benefits that will occur, but you didn't ask for these.
Re merging ISO9K & 14K - suggest you do this only where the standards have common elements such as mgmnt review, document control & recordkeeping. Eventually you can do more harmonizing but its easier to understand 14K if you put it in beside, not inside, your existing system.
