OP, each rescue varies in what they ask as an adoption fee. When you adopt a greyhound, they will usually be neutered/spayed, chipped, vaccinated, wormed, health checked and assessed for behaviour. Most rescues have to fund raise to cover all the costs as the adoption fee rarely covers all that I've listed. It varies from rescue to rescue - anywhere from £80 to £180, but you should be looking at around the £100 mark as a minimum. The adoption fee also means that you will get lifelong backup - I know of cases where rescues have provided support five or six years after adoption, for example, by providing emergency foster care.
You should also be budgeting for a suitable collar, winter coat, muzzle, vet bills, food, kennelling for holidays, insurance, worming, training classes, safe transport in your car, and bear in mind that greys often need ongoing dental work which is rarely covered by insurance, and can be expensive.