What they can afford. It is impossible to put a dollar figure on that, because it will vary so much with the couple's circumstances.
There's a huge difference between the RING being cheap or the GUY being cheap.
A very young couple with little or no income would have a small home wedding, and can use a $2 chainstore ring, just symbolically.... and maybe plan for a good eternity ring in 10 years' time if they make it that far. If he spends his whole unemployment benefit on a ring, then borrows from her for food the rest of the week, there's a problem.........
If a guy with good income, from a well-off family, buys a small cheap ring for his girl, she should look back at their relationship and see if he was cheap with outings, dinners etc as well. Another problem.........
If someone on an average income goes into debt to buy a ring, problem again. Bigger problem if she expects him to............
If they're not already living together, how much to spend on the ring is the first joint spending decision a couple makes. Followed by what they will spend on the wedding. How they discuss it, how they decide it, will depend on their attitudes to money, budgeting and setting priorities.
Honestly done, it can be the make or break discussion for the relationship, if their expectations on this don't match up. If he turns out to be cheap and mean, or she turns out to expect more than he can provide, this is going to continue into their whole relationship forever (or until the divorce court).
Of course if both choose show over substance, and are happy to go into debt for a ring they can't afford, the marriage might still fall apart, but at least not because they feel put down by each other's attitude to spending.
All that said, estate or heirloom rings will often be a much better ring for the same money...in many places used items don't attract the same sales taxes as new, leaving more money for ring quality. Estate also means a more exclusive design, as even if they were mass produced originally, the others are no longer around.
Worst choice is something from the internet: even if it does arrive there is no guarantee of the quality and no comeback if it doesn't suit.
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A young couple I knew, living together from when they were unemployed and couldn't afford a wedding, used to buy a pretty ring with real stones from pawnshops, whenever they had a few hundred to spare. Planning their engagement ring was an expression of commitment that went on for years. Ten years, 2 beautiful sons and their modest own house later, they could finally afford to have those gems made into a ring of their own design, the leftover gold paying the jeweller for the work. She had the satin wedding gown of her dreams, they had a reception for all their friends, nobody left out....but made it a simple barbecue at a local hall with minimal decorations.