Agreed with @metadat, more information would be helpful.
Simple rule of thumb is the most you could expect would be around 12x proven revenues or savings. A strong negotiator would probably force you down to around 6x - 8x range.
The high costs to build could be a reason that it hasn't been done before. In most organizations, the business case upon project inception would have to show a significant gain from the cost of any project.
Another concern would be that your current employer might lay claims upon the results of your side-project if The Thing is in a related area. Of course, that depends upon your employer and the jurisdictions you are operating within.
