Updated: Oct 1, 2018
Why do NSDAP membership due stamps differ in price (and stamps color)? What dictated one price for someone and different for others?
I found explanation after studying "Organisationsbuch der NSDAP", 1943 edition, page 527.
There were 9 groups numbered from 1-9 (in Roman numerals).
Group I - for members who joined before 1. April 1933 only
Members of mandatory or voluntary work service
Members who can prove they live in poverty
Dependents (e.g. students living with parents)
They would pay RM.1 a month.
Group II - also for members who joined before 1. April 1933 only
All that don't qualify for Group I. They would pay RM.1.50 a month.
The groups from III-IX were for the members who joined after 31. March 1933. Their dues were dependent on the monthly family income.
The same criteria and fee (RM.1) as for Group I.
Group IV - IX
Members with monthly family income of
up to RM.200 (inclusive) - they would pay RM.1.50 a month (Group IV)
up to RM.400 (inclusive) - they would pay RM.2 a month (Group V)
up to RM.600 (inclusive) - they would pay RM.3 a month (Group VI)
up to RM.800 (inclusive) - they would pay RM.4 a month (Group VII)
up to RM.1000 (inclusive) - they would pay RM.5 a month (Group VIII)
and finally above RM.1000 - they would pay RM.8 a month (Group IX)
If both spouses were the members, they would not have any discounts. Each of them would be measured against criteria above. The same for children of the members. For example a man is a director in a factory with an income of RM.1000, his wife is a house-wife and child studies. They all join NSDAP in 1934. Spouses would each pay RM.5 a month and their studying child would pay RM.1.
There was also a discount for particularly severe hardship that would allow a member to continue their membership with contribution of only RM.0.50 a month. This was a special case reviewed and approved (hopefully only temporarily) by a local NSDAP official.
To the left is an example of RM.1.50 due stamp (Group II and IV) and below is a monthly stamp for Group V (RM.2).
And what's the extra RM.0.30 every month?
NSDAP Hilfskasse was a fund for support of NSDAP members who fought and suffered (death or injury) for the cause. This was mandatory donation fixed at RM.0.30 a month. Before 1939 had a separate stamp, while with the 3rd issue stamps, it got integrated with monthly dues stamp. Membership books issued before 1939 have 2 columns for each month - dues and Hilfskasse. 1939 issue of the book did not have separate columns for Hilfskasse anymore. Pre-1939 Membership Cards (for the first year of membership) did not have space for Hilfskasse stamps. Instead a dedicated card was used.
To the left is the last 1938-issue stamp for just RM2 of dues. Below is the separate stamp for monthly donation of RM.0.3 (Hilfskasse)
Books edition from 1939 rid of "Spende" column and there's only one for combined due + donation stamp.
In 1939 due stamps for quarterly, half-yearly and even yearly dues were introduced. They also included RM.0.30 "Spende" already.
Quarterly membership due stamp for RM.5.40 (which is 3*(1.5+.30)) - red, like the equivalent monthly stamp.
Similar variant for Group V (green, consistently with monthly version) is below.
There's whole "NSDAP Collection" where you can find more items related to the Party.