Note that there is a visa fee and a departure tax for Jordan. Both are payable in cash, and I think the departure fee only in Jordanian Dinars.
The one-month on-arrival visa is 10JD, while the departure tax (land and air) is 5JD. I paid my visa fee in Dirham I had left over from Dubai and I’m fairly sure they accept US Dollars as well. This is handy as there are no cash facilities around the immigration area, so if you don’t have the money on you then you’re screwed.
When leaving, ensure your passport is stamped and you pay the fees. The whole procedure I went through was really badly organised and I almost missed my bus when the driver realised that the staff in immigration hadn’t sorted my passport out. Again, there are no cash facilities at the King Hussein / Allenby Bridge location, so ensure you have enough on you before you arrive.
These details are for British passport holders, because I am one. Always check your own details shortly before travel.