Sossusvlei was the most expensive stay during our time in Namibia, so if you can get a spot at Sossusvlei Camp I would go for it, I think Sossusvlei Lodge also offers a few campsites.
If you find something cheap it'll be quite far away from the entrance, most people want to go in as soon as the gate opens to see the sunrise, if you're staying at the lodge you can go 1 hour earlier.
I know just get up early and drive there, the problem is you're not allowed to drive a rental car between dusk and dawn, if you do and have an accident your insurance is void, I've read about people that couldn't leave Nam until they fully paied for the car, all rentals have black boxes that record time and speed and it's the first thing the insurance wants to learn about an accident, that's why I would stay as close as possible even if it's more expensive.