A custom ring will cost more than one you pic out from a jewelers case. There are thousands of selections. If you go into a jewelry store and say you have $5K to spend, you will spend every cent of that and perhaps even more with taxes and warrantees. If the store knows you have money in your pocket, they will pressure you to buy a ring right then and there. So, when you first go shopping, do so without buying anything. You know, window shop. Just see what is available and at what price. If they ask a price range, at first say you are unsure because you have no idea about prices. Educate yourself by asking lots of questions.
If you see a ring you love, hey, it could very well cost $3500....saving you $1500.
So, shop around and do not feel pressured.
I have seen some absolutely lovely engagement rings in the Estate Sales case of a jewelry store. Do not overlook antique stores as they have some truly beautiful engagement rings. Do, however base your sale on getting it evaluated by an independent jeweler to be sure the stone(s) are real.
Go to a family jeweler, if possible. We have a local jeweler that has four stores and they have been around for two generations. So, look for longevity and do not fail to seek out internet reviews/yelp.com on ANY store you deal with.