You might struggle as it's the height of season, but when we went to Barcelona and decided to stay the night, we had no problem getting a room last minute. This was in y2k, and we stopped on Las Ramblas and it cost us £13 each for a double room. It was "quaint" shall we say, but lovely. There were other hotels vacant, but they were more expensive. We are going back to Barcelona in July and staying 2 nights. Hotels seem to be £120 + altogether for the 2 nights for a double room. You can probably get them cheaper.
Sorry, can only help you with my experiences in Barcelona. PS: that was for the both of us combined price

Formula 1's are fine - they are cheap but clean and easy. Yes, they really -are- that cheap.

We have just stopped in at b&b's or hotels for overnight stops on our way on the bikes through France and haven't had a problem. As for apartments in Spain, not so sure. You can stay for one night, we have done so in Calella, but we booked it in advance (about 3 days before...). I'm sure there are so many apts, you'll find somewhere to to stay...

Cheapest.. not sure, but we paid 24 euros each including breakfast at the president in Calella for one night in July. We've paid 80 euros for one night in a nice hotel in Barcelona that we booked the night before. I think in Lloret, we just turned up one afternoon and got a room for about 40 euros for the evening. Beats driving back home!

Best of luck. Remember that petrol station in france on the smaller roads have -stupid- hours and even though they say they are open 24 hours, that's only if you have a french bank card to pay through the machine.

We have done the run from St Malo to Costa Brava a few times, there's some nice places along the way. We stayed overnight in Cadillac once - that's a beautiful town. Also recommend Foix as a nice place to stay with some enjoyable roads leading up to it...

Have fun!