I think it would be best if Shadchanim state their fees at the outset. I think $1500 from each side is more than fair, but each community may have their own norms. The gown rentals don’t take “whatever the renter thinks is fair”, and neither does the caterer, the band, or the wine store. It is fair to the family to know the price at the outset, so they aren’t surprised that they “have to pay” for the service. The price should be set based on the average amount of hours invested in a successful shiduchim (yes, there is great variability, and yes, and average can still be determined by a veteran shadchan), and on a reasonable fee per hour.
Someone who feels they don’t have the “extra money” lying around should also question themselves if they have the “extra money” lying around for a band or a caterer. Each family can determine if they are interested in the services of a shadchan, and can decide they they are not (either relying on friends and family to red shidduchim, or not going the formal shidduch route at all), just like someone can decide that they are making a wedding in their backyard to save the expenses.
In my own dealings with shadchanim, I have been surprised at the amount of time they invest in each attempted shidduch, and feel bad that conventional norms do not have shadchanim being paid for unsuccessful attempts (although I am sure they appreciate the occasional thank you note or other small token).