I think I understand your rationale behind a "one time" membership fee - that being: users would be more inclined to sign up since they only have to pay once. Not sure that's the best approach. Have you sat down with a calculator and thought about this?

Think about renewable membership. Let's look at a scenario based on a fee structure of $25, $50, $100 and $140. Lets assume for the example that you have the same number of members sign up each month in the following breakdown:

2 @ 140

4 @ 100

6 @ 50

8 @ 25

This totals $1180 a month. In a one time fee basis, over the course of two years you would make a total of $28,320 ($14,160 per year).

Now let's say for "renewal fee basis", you again still have the same number of new member sign ups each month consistently for your first two years. And let's say the second year, only half of your first year's members renew at each price point. Your second year you would gain an extra $7080 making your two year total $35,400 instead of $28,320.

Which would you prefer?

In theory this could compound year by year. The key is making your services good enough and attractive enough to both get new member signups in the first place and secondly keep them wanting to come back. It isn't the pricing or renewals. It's the service you provide that makes the difference.