They start at 25k. There seems to be no difference in how much rank you get between 25k and 1 million so always look for 25k ones.
You can also do a trick.
Get 19 missions.
Take a 25k (or any number) donation mission as your 20th mission.
Take one more mission. You'll get the "Too many missions" message.
Turn in your donation mission.
Try taking another mission (any). It will say that the data is inconsistent or something and reload the missions.
The 25k donation mission is now back.
Take the mission again and repeat until broke. Actually, the donation mission will vanish after a while when the missions reload server side. Depending on lag ("STAND BY"), you can do quite a few before it vanishes naturally.
I thought this was an exploit but its been reported and we've been told its OK and by design, otherwise I would not have said anything about it here =)
https://forums.frontier.co.uk/showthread.php/269503-Infinite-donation-missions
Going from neutral to Duke with Empire costs about 80 million doing this, but mixed in a few 50 and 100k donation missions as well. Thats about 3-4 long range hauling missions so quite cheap anyway.
Plus, its boring as hell.