Botox can help delay the landslip as it works on the supporting muscles, but it can only do so much. At a certain point, too much volume under the skin is lost and botox can't fill the gap - that's where fillers come in (or of course surgery).
So the whole cost/benefit thing really depends on how your skin is now.
Sure people can give you an idea from what they have experienced, but getting a consultation from a Dr, preferably one who has been personally recommended to you and whose work you can see, will give you the best guide.
OK, so there's the base advice. Also: sleep, sunscreen, excluding any medical issues, dental work, avoiding stress, blah blah.
Any injectable will depend on your own metabolism and generally appointments can be further apart over the years of use. To start with, botox or similar is done about 3 times a year, more or less, so multiply any quote by the repeats. Fillers can last anything from 6 to 18 months, depending on placement and product.
Neither should be done on the cheap, they are still procedures so a small amount done well by someone who knows what they are doing will always be better than shit tons by someone with little clue or qualifications. And neither might be right for you (with loose
For cheaper results, consider microneedling? It's not terribly pleasurable apparently, think the rollers are cheap from amazon though.