It's very difficult to determine whether one solution will save money over another. There are many hidden costs beyond licensing costs. Lotus Notes is not very similar to Outlook/Exchange other than they both can be used for email. You could drive yourself mad trying to come up with a fair assessment. If the decision is above your heads, I'd just get ready to deal with it. If you have some say in the matter, consider the following.
First, there are many tools that allow you to have both Exchange and Lotus Notes in coexistence. You might consider a phased approach to moving between platforms. I've had success in a small company running Lotus Notes side-by-side with Exchange for years.
Second, there is an undeniable cost to moving between platforms. Email is the simplest to transition and still could be very expensive given the down time, time to retrain staff, loss of email or fidelity of email during transition, etc. But beyond that, moving applications/data from SharePoint to Lotus Notes is an enormous and expensive undertaking.
Third, I don't have numbers to back this up, and I'm going from gut instinct here, but there doesn't seem to be a plethora of Lotus Notes / Domino professionals out there. On the other hand, Microsoft Exchange professionals are much more common
Lastly, consider the worst case scenario. What if Lotus Notes loses market share in the next five years. Some might say it has already lost the email battle, but current reports as of 2010 are stating they're a close second to Exchange. Consider which company you feel safer with in the long run.
Just to throw some love Lotus' way, and to make you feel a bit more at ease if you're truly stuck moving, I have to say I really like Lotus Notes/Domino for what it is. Its strength has never been email alone, but as a collaboration platform and rapid application development platform, it's great.