To begin with all pure honey is good for you and has antimicrobial properties.
The reason for this is that when honey comes into contact with a wound it produces hydrogen peroxide and hydrogen peroxide will eliminate a range of bacteria.
Where Manuka UMF (unique manuka factor) honey is different and superior is that also contains a compound called methylglyoxal and this compound is highly antibacterial, able to eliminate bacteria that are resistant to almost all forms of antibiotic including MRSA.
The level of concentration of methylglyoxal is described in the UMF certification which typically starts at 5+ and can reach as high as 25+ on very rare occasions.
For example the concentration of methylglyoxal in Manuka UMF 15+ honey is approx. 515mg/kg
The concentration of methylglyoxal varies from season to season depending on the density of Manuka plants in flower and their production of dihydroxyacetone (DHA) in the nectar which the bees convert to methylglyoxal.
The price of Manuka UMF honey generally reflects the UMF certification so the higher levels of UMF like 20+ will be the most expensive because it has the highest levels of antimicrobial activity.