I'm planning to go to Germany next year. A direct flight from my home country (Dominican Republic) would be about USD $1400. I have B1/B2 US visa and I was wondering if it'd be possible for me to purchase a round-trip flight from DR to Miami and then a second round-trip flight from Miami to Frankfurt. That'd save me up to $500 dollars, which is a considerable amount. I'd like to know if there'd be any issues with me entering the U.S twice in such a short period of time (around 10 days).
I believe you will not have any issue. I can say because my experience. I have been in US during February and I leave in February 15th. So, I traveled again to US 2 weeks after and I did not have any problem. Also, I do not think that they will make difference if you buy a Round-Trip from DR to Frankfurt or if you buy two Round-Trips. We know that there are a bunch of flights to Canada that there are legs in US. It is important to know your situation, if they ask you and you need to know how to explain it.
You can do this but if your first flight is delayed you can run into serious problems, since it's not a single booking. Re-booking your second flight on the same day could be quite expensive.
I think your $1400,- number is too high. Consider alternative options using budget carriers. In Spring 2018
Eurowings offers Puerto Plata<->Cologne round trips for as little as $600,-
Air Berlin has Punta Cana <->Dusseldorf for about $1000,-.
Condor offers Punta Cana <-> Frankfurt for about $820,-
Added benefits: these are non-stops.
Disclaimer: these don't go every single day, so a bit of date flexibility will help. Some of these can also be combined with other cities in Germany. Air Berlin will take you to Berlin (from DUS) for only a few bucks more.