Above about $1-2m enterprise value, any half serious buyer will require exclusivity as part of an LOI. In fact, it is a good signal of an unserious buyer if you get an LOI without a binding exclusivity clause.
Extremely rare at the kind of enterprise value I see ($5-50m mostly). From what I hear also extremely hard to enforce (clause is usually very vague).
Even in deals where there is a breakup fee, it's not a part of the LOI. It would be negotiated as a part of the Merger Agreement or Asset Purchase Agreement, to cover what happens if the deal signs, but then one party breaks off before moving to closing. The LOI is always non-binding so there's no firm commitment to do the deal at that phase and no penalty for breaking up.
Any good resources for determining SaaS valuation, outside of the conventional methods of analyzing the value of a business? (DCF, etc) ?
Beyond talking to a knowledgeable M&A advisor who does deals in the range you’re looking at, I’d take a look at usesummit.com (full disclosure: am an investor)