My post was ambiguous because there are at least two "costs" that I could be driving the breakup fee (BUF).
The primary and dominant "cost" would be a change in law that deprives newco of the tax benefits of moving to Ireland. That founded my argument for a LOW number. Other costs in the long run become rounding errors. Major tax changes are beyond the control of either party so they provide an easy out, a low BUF.
Of second order interest is the need for a significant BUF - "cost". Here the banker and legal fees are substantial and seek to protect AGN - the non-driving party.