I (personally) think that if you're going to sell design, you need to sell design. $150 and a sketch you spent 2-3 hrs on is an extended consultation fee, and if you're then making a second trip to present you've just broken even. Maybe. If your rates are low.
Unlike AGLA I am not a landscape architect and I did not receive advanced education in this. However, I also make my living as an independent landscape designer, doing designs for both homeowners and landscape contractors. There's no one single path. BUT, we're both selling the same thing: solutions that work, that are appropriate to the site and the client, and that are cleanly presented and easily interpreted.
If you're going to charge more than a couple of bucks for design fees there are certain things clients are going to expect, and your presentations have to be top notch. That means well drafted plans (hand or CAD), photos of recommended plants, etc. If you can collect $500+ for a pencil sketch on graph paper and a handwritten estimate on three-part carbonless forms and do it week after week? I salute you and will buy you lunch if you'll teach me. I don't see it happening.
As for when to charge? You have to know your client base, but I personally do not put pencil to paper without getting paid. It doesn't matter if it's a $100k backyard oasis or a front foundation planting. I make that work because I have sold the homeowner on the value of my knowledge and experience that I've gained by working in this industry since I was 16.
If the argument is "well everyone else does free designs", well, that's fine if you're no better than everyone else. When I'm on a sales call and someone asks "well why would I pay for a design, I've had two companies give me plans and estimates?" my response is always "if you think their plan is the absolute best way to go for what you want, you don't need me. But my job as a designer is to come up with the best plan for your needs." I don't negotiate, I don't back down, and I usually get the job. So the other companies not only wasted their time for nothing, they're not even going to get a crack at bidding the job because when we're done, the homeowner trusts me and anyone I recommend.
There are still a LOT of guys out here who give it away for free. I got a call from someone who had already talked to several pool companies and several contractors who had given them drawings. I almost didn't go but I pre-qualified them hard and I had a slot to fill in my schedule anyhow. Well, after all the folks they talked to they signed a full-price design agreement. Why? Because every one of the "free" guys were trying to play it smart and only come up with rough concepts and ballpark pricing, saying "sign the contract for construction and we'll figure out the details as we go." My clients wanted to know how it all would work together, how it would look, and exactly what it would cost, BEFORE committing to build. This is why they are now my clients, and it's what you should be selling when you sell design.