If "md" means "billable day", a $600 billable day is extremely low for this kind of work; that's closer to what people pay for network pentesting. $1500-$2000 is closer to the market (before discount, assuming senior but not principal level delivery).
When I worked as a 'consultant' (glorified contractor) .Net developer, the company charged > 90 Euro / 105 USD per hour for my time. So that would make my going rate be > 800 USD / day. This is in a country where 50K / year is a decent developer salary.
I do not believe you can find pen testers worth their salt who would cost _less_ than a non-distinctive developer. At least not one who will do more than run some automated report over all your endpoints.
Calc based on 3 months, 5 people, 600USD/md rate.
EDIT as I can't reply to tpaceck below: no, those 2000usd/day rates do not exists in projects in size of 300MD like here. In general they do not exist for big projects.
Yes, I agree, you have rates around 1200 in high cost countries, yet as I wrote earlier, you can have similar/the same skill level at 600 usd/md if you're willing to work with guys not from HCC.
As to the skills I'm talking this level: https://research.securitum.com/mutation-xss-via-mathml-mutat...