That's like asking how long is a piece of string.
One of the things you will find is that the costof oil varies greatly between suppliers , as does the quality.
Good suppliers are : quinessence, fragrant earth , absolute aromas, materia aromatica, shirley price but this is not an exhaustive list ( I'm sure others have their favourite suppliers) and each of them has differnt prices. - Put these names into google and you will find there website addresses.
One thing you will learn on your course is about adulteration so it really pays to buy from a reputable supplier who will have done their best on your behalf to give you a good quality therapeutic oil.
I suggest you buy something like lavender from each supplier as you need it, and you will use lavender most I would guess ( I know I do), and do some sniffing to find the one you like the best.
You Don't need to buy all your oils at once - buy them as you need them and learn about them , that way you get to know their individual qualities.
Oh and enjoy - this has been one of the most fulfilling therapies I have learned from a purely self help point of view.