First of all, most new complicated phones will not be unlocked for RD$200, more like RD$1000 or more. N-series Nokias, Iphones, new complicated Samsungs, Motorolas, Android phones, etc. All those will cost much much more than RD$200 to unlock.
Back several years ago when I bought Nokia N73 from Orange, nobody wanted to even touch it in Higuey! Not even for 1000 pesos! One technician asked for 1000 pesos but later on changed his mind and refused to touch it. I had to have it unlocked in Europe for around 40 Euros.
Viva dealers will unlock many phones for free when you activate a new SIM card with them (it costs 100 pesos to activate and comes with 100 pesos calling credit, you need a passport). So you basically get free unlock and free SIM. They may refuse to unlock certain complicated phones if they fear they might damage the phone and/or have no experience unlocking that particular model/brand.