There are no set acquisition costs. These will vary per site, source and campaign. To best understand how much you can afford to spend you should calculate IRPB and TRPB...
Initial Revenue per Basic (IRPB)
This is the amount of Initial Revenue within a selected time period divided by the number of Basic Registrations within the same time period.
IRPB will give you a rough idea of how much you can afford to spend on each registration.
Of course, as members continue to re-bill you can afford to spend more per registration. Therefore, when looking longer-term a better figure to use would be Total Revenue per Basic...
Total Revenue per Basic (TRPB)
This is the amount of Total Revenue within a selected time period divided by the number of Basic Registrations within the same time period.
This takes into consideration the Re-bill Revenue being generated by the members.