Here's one way to look at it:

Basically it depends on the expected return you will get, and how quickly you can build up the amount to invest to a larger sum.

Fundamentally, you expect money you invest to appreciate (even though it may not do so uniformly, in the long run this should hold true). This means that the longer you have it invested, the more you expect it to grow.

Basically what you want to do is to find the "point of indifference" where the amount of gain that you sacrifice by waiting to invest equals the cost of investing immediately.

Let's do an example:

- Let's say your expected return on an investment is 4% per year

- Let's say you currently have $1000 to invest

Basically your $10 commission represents a 1% fee - because you expect to gain 4% per year, the 1% represents 1/4 of a year's gain, or 3 months of gain.

Let's say you know that you will get another $1000 in a month.

If you wait until then and invest the $2000, you're now only paying .5% which is 1.5 months of expected return.

So by waiting 1 month, you've managed to reduce your "lost" time from 3 months to 2.5 months from the initial investment opportunity - this is a win.

The higher the expected returns, the shorter the time windows become.

This is all pretty complicated, so ultimately you could also just use a rule of thumb - find what percentage you're comfortably giving up in the first year of your investment and wait until that threshold is reached. If your threshold is 1%, that number is $1,000. If it is 0.5%, that number is $2,000, etc.