Basically, conversion rate is goals divided by total visitors and multiplied by 100 to make it into a percentage.

So if you have 100 sales and 10,000 visitors, your conversion rate would be: 1%.

You need to measure 2 things in your case:

1. Conversion rate for each plugin.
2. Overall conversion rate for all sales.

2% estimate is a very, VERY broad benchmark marketers like to throw out there when asked for average conversion rate. It will be wrong.

Here's an article with average conversion rates segmented a bit, it is still not 100% accurate but it might just give you a bit more accuracy.
http://www.smartinsights.com/ecommerce/ecommerce-analytics/ecommerce-conversion-rates/

Re-reading the question, I get a feeling that you might be trying to do some planning, for business/marketing plan.

It doesn't matter how many members you have, because not everyone will buy and you will also have new visitors converting to members as they buy plugins.

Now you might need to segment your conversion rate:

1. Members
2. Visitors
3. Plugins

Keep in mind, each plugin will have it's own conversion rate so applying 2% rule for each will be wrong. Some will be popular, other's won't.

So.

If you want to use 2%, that should be your overall sales conversion rate. Then, you need to guesstimate sales for each plugin within those 2%.

Let's say:

You have 10,000 visitors, 2% conversion rate would give you 200 sales.

Now:

- Plugin A = 50 sales = 0.5% conversion rate
- Plugin B = 100 sales = 1% conversion rate
- Plugin C = 30 sales = 0.3% conversion rate
- Plugin D = 20 sales = 0.2% conversion rate

These are random numbers, you need to figure out which ones might be more popular than the rest to put some numbers on them.

Keep in mind that's for all visitors, members and new visitors. You can segment that to members by looking at how many sales were made by members, rather than everyone.

This is just basic overview to help you think it through. Hope that helps.

From the question it sounds like this is more about forecasting than it is about tracking but I could be incorrect on that. If I were tracking something like this for a client I would generally track a conversion when any item is purchased and then I would track the conversion value as the dollar amount of the actual transaction. You could also leverage Google Analytics e-commerce tracking to get good, detailed numbers on the items purchased and amounts of each item and of the total transaction.

Based on this logic your logic and a conversion rate of 2% you would have 2 conversions out of 100 visitors and a totally average sale value of $x where $x is the total value of the conversions added up and then divided by the number of conversions. If sale 1 was $50 and sale 2 was $100 then you would have a 2% conversion rate with an average transaction value of $75.

One option is a mouseclick software whereby you can track each click and where they are clicking or they have a heat map option. If I had 5 different plug-ins to purchase and I could track total unique visits to the site and the purchased amount of each one, you could easily come up with the answer. EX: 1,000 total unique visits, 100 purchases with an equal distribution of each plug-in means 20 were purchased of each. So 2% bought each plug-in but 10% converted total.