I save in cash and purchase in blocks of a ~$5000 minimum. That way, even with the full price online brokers transaction costs are less than 0.5%. When I started out, this means I didn't buy all that often. As my income and dividends rose, I'm now saving/investing ~$10k/month, so monthly purchases work out OK brokerage rise.
If you have a low balance, beware the minimum monthly spend with IB. If you have a really low balance it may be an impediment to you.