There is an advantage working on Fiverr (I am a developer in WordPress & Odoo). Like you mentioned that overheads are not there. There are two ways of looking into the same.
I come from a world that is outside of Fiverr. So my understanding will not be as good as other veterans on Fiverr. But here is my take.
When I run my numbers, the cost of my overheads (Website + Marketing expenses + Time/Efforts) is about 35%. Fiverr takes about 20% as commission. So, I am still saving 15%. Now, the logic for me here is to pass this 15% that I am saving to my clients on Fiverr.
For me each project will still have some assigned cost like a development server (usually a VPS). These cost are still built into my costing.
The second way to look at it is that when people who are coming directly into Fiverr they are saving approx 30% that they would have otherwise spent to get a order or a contract. Also, when the eco-system sees your reviews etc., your account will snowball into a bigger account. In the world outside, each clients the whole pitch and time to close is much longer.
When all this is calculated passing on those additional benefits to your clients by lowering your price is win-win.