To upgrade or not
The thing to think about is time as well as cost. Look at the benefits and features that you would gain by spending the money. How many credits would you gain by upgrading? How much time would it take to free surf that many credits? If it takes you an hour to generate 100 credits and you can get 200 for $5.00, your time should be worth $2.50 an hour, right? What are the results you're getting from the sites as a free member. If you're getting decent results from the free credits and by upgrading, you'd get 200 or 500 or more credits, it's worth it. Some of the sites even give a shorter timer for upgraded members or put members under you if they join without a sponsor.
Happy surfing,
Brian