First of all: Go to the spanish/german/Irish/Belgium website of Hertz too and compare the price to the UK one. i'd say 70+% of time it comes out cheaper.
I ALWAYS book economy car at LAS cause 95+% of the time they have LOADS of adrenaline cars available (october-march). Like everyone said here: usually between 30-80$ per day depending on what car you want and how good you can negotiate (never accept first offer and always ask for manager after counter offer and be very nice). Also at LAS there is one particular girl working (downstairs at gold in front of the board with the names) who is always the friendliest and gives you amazing (best) rates. If you want i can DM her name to you and you can ask for her when you get there.
Also use the Platinum AMEX CDP which always gives you 20% discount aswell when pre-booking. Never been asked for it in 25+ bookings so safe to do so!