Hi Margaret.
Been on 7 day Nile cruise + 7 days stop in Luxor earlier this year - not with JY that time but will be v similar for you.
You will not need anything like £400. Eating out is generally cheap. I was hard pressed to spend £10 on 3 course meal with a couple of beers. Don't be afraid to go outside the hotels which are much dearer.
You can pay with ccard on the boat. If you buy drinks etc on board those are v dear eg. £3.50 for a bottle of Stella. Wine similar. Water also v dear on boat and hotels. Buy water (and you will need to take some on trips)from local shops much much cheaper take on board. Be careful where you eat and don't drink anything other than bottled water. Don't use tap water on boat other than to clean teeth. Take some insect and mosquito repellent. Not as bad as I thought it would be and will be a lot cooler now than when I was there - 45C! most days. Much cooler now but you will need sunblock and hat + loose clothing.
I took no Egyptian money at all just a few UK£s. Also got visa at airport after landing. They have banks in there where it is much cheaper than if the rep gets it. Needed £10 UK for that and got change. Get some Egyptian £s when you get there from hotel cashpoints or local banks which will give you a better rate than here usually. You will not need much. Be sure if you hire horse carriage or taxi you AGREE a firm price for it before you set off and that it is Egyptian £s not UK £s as they will try to confuse you.
Just smile at people hassling you to buy stuff, hire taxis or carriages. Keep walking and don't make eye contact. OK when you get used to it but it happens everywhere.
You will love the trip. Try to talk to the locals they are lovely and very poor generally thats why they hassle you to buy etc.
Usually you don't tip on the boat until the end and then just give in an enevelope to be shared out by all the boat staff. They get v poor wages so rely on tips. You can tip the Egyptian temple guide similarly at the end. You will need a few Egyptian £s to tip coach drivers etc - £2 Egyptian which is only about 20p or so is OK.
I had a tooth filling fall out while I was there. Had it refilled for £10UK + £2UK tip by local dentist - good job! You can get medicines easily and no prescription needed. Should you ever need an injection make sure you take your own sealed hypodermic as I wouldn't trust the ones they use which are re-used. Hygiene is NOT brilliant in Egypt. Take small bottle antiseptic hand wash or wipes to fit in handbag and use when you handle money etc as this is often the cause of dodgy tummys. No one had a bad stomach at all on my boat and you will be fine if you just take sensible precautions about water and what and where you eat.
Enjoy the trip! You definitely will.
John