Hi @David Ward, this is a great question since so many people are looking at either SFRs in the burbs or larger 40+ unit rental buildings. Boston is extremely unique because of the limited land, housing stock, and very high prices.
Being a developer and investor in Boston, the only way a property is going to be a "good deal" is if there's a value-add play. Otherwise, many view Boston's market as a safe haven for their money, hence the low cap rates as noted by @Jarrod Kohl. That formula you mentioned just does not work or even come close. Another reason 3 family buildings in particular will sell for a premium is because they are usually converted to condos and they are the easiest to acquire. Even 2-3 years back, the prices of triple deckers in Eastie were 30-50% less.
In terms of what you should do.. it's a tough decision and completely up to you. Personally, I don't like to put my money into something I can't control - yes the stock market has been great, even during COVID, but I would rather put my money into something I can manage (even if not 100% due to moratoriums and zoning risks) versus a stock that I have no control over and is basically a legalized form of gambling. I still feel this way even if that means making less in hindsight.
In terms of buying something recently, we put a rental building under contract back in April and closed in July - it's a value add play where we get the existing building but it comes with a lot which we plan to subdivide and develop. We did this strategy last year too and are just about to go through the ZBA process on that one.
So in terms of what 100k can get you right now in Boston -- yes, you're limited on your own. IMO your options are: (1) look in less expensive markets where your dollar goes further, (2) invest in a different asset type, (3) invest with someone else / partner on a deal, (4) wait for prices to adjust, if it happens... personally, I would expect to see some distressed sellers who are going to be taken out by the eviction moratoriums due to abusive tenants - MA is brutal when it comes to the super-progressive lawmakers who think free rent w/o aid to landlords is perfectly acceptable... but I digress.