Should upsells cost more or less than front end offers?

Both. An upsell for people who have the money, and a downsell for the people that don't.

In my opinion I think it works both ways, hence why you hear both ways,
In fact you're probably going to hear both sides in this very thread.

It's really more about the value or perceived value of the product your upselling

some people are going to say it needs to cost less because that is where they have seen success. While others are going to say it has to cost more...because that is how they saw success.

I think this is more of something that needs to be determined on a case by case basis, and I'm sure there are those who will disagree with me.
It's really all about what the customer thinks your upsell is worth, and how well you can convince them of that worth. If you're selling junk above your front end product your gonna lose loyalty, or increase the number of buy backs.

If the upsell only adds value to the front end product, like an extension for some software or a pro subscription, then it is probably safe to say it should be less, as the upsell is included in the price of the front end.
But if the upsell is a 33 part video course that goes along with the eBook that they just purchased for $27 then I think it should be more than the front end product as it has more value, and adds more value.

That's just my opinion, I hope you understand what I was trying to say, and hope this helps answer your question.

In my opinion it has a lot to do with the product you are trying to sell as a back-end offer. It is all about perceived value and what is it worth to your customers.

In my experience both lower and higher priced back-end offers work. What matters the most is WHAT you offer as a back-end offer rather than the price. Usually, the best converting upsells are the ones that complement the main offer by either making it easier or faster to get results from it.

Opinions are just that - opinions... and everyone has them.

Only way to know for sure for YOUR business is to test different price points until you find something that gives you the highest additional revenue per buyer.

I've seen both done with great success. Back when I used to sell my own stuff, I did both with success.

You won't know until you test it with your own buyers.

Upsells should cost whatever the vendor decides on it is their product after all no-one is obligated to buy anything.

But I do think what you buy on the front end should be a complete product not having to buy the upsell to make what you initially bought function.

It all depends on what the offer is etc... Usually upsells are higher priced because the vendor wants the buyers to purchase the product first lower end offer means more front end sales.

It depends on the product you are selling. If your upsell is something that they really want, and it benefits the buyer, then you can use a higher price. If it's something smaller, you can use a lower price.