Can you define what you mean by inexpensive?
Ms. as219 and I are leaving for Egypt in a week. Her first time, my, well, I lived there for a year. I've done a half-dozen cruises. After a lot of research, we opted for a "dahabiya" (sail/house boat with
Nour el Nil. It's not inexpensive relative to other options, but since I don't know you're definition it's hard to say. One thing to keep in mind re price is that you very,
very much get what you pay for. The Nile and the temples are, of course, the same no matter what boat you're on, but if you don't sleep well or get sick from the food, it's a little harder to enjoy. That said, when my parents visited years ago, I think the 4D/3N cruise cost $300 p/p. It was decent IMO, but they still say it was the best time they've ever had traveling.
In terms of last minute booking... It used to be as long as you were there in the shoulder- or off-season, you could absolutely do this. Once I walked to the dock, put $100 in the guy's hands, and my then GF and I had a 3D/2N cruise for $50 each! This would have been impossible in the high season, though. These days the stories I'm hearing are mixed. Yes, tourism is suffering, but that has led many boats to be mothballed. So on a given day, there may not be many boats sailing. If your schedule is flexible, this might work; if not, you could be out of luck. Our sailing date for later in February is already sold out. It's a small boat, but still. I would say that unless you (a) have a lot of flexibility and (b) are ok if it doesn't happen, then sure, book same-day. Otherwise you might want to think about booking in advance.