Choose a completely different domain
It's easy to get attached to a name, but it's cheaper and faster to abandon the name once you find all domain variations are taken. This makes sense especially if the social media names for your preferred domain are also taken, because these may be even harder to acquire than the domain itself.
Choose a slight variation
If you're selling an app called "monkeys" and
monkeys.com is taken, try
getmonkeys.com and variations. But be careful not to infringe on trademarks. (Do a trademark search.)
Offer more money
Pay as close to the asking price as you can afford. Raise your offer and make it clear that you can't go higher. Be prepared for them to reject it.
If the name really means that much to you, you might just have to pay the asking price.
Buy it later, when you can afford it
Choose a different domain name for now. If your site takes off and you still think it makes sense to adopt your preferred name, rebrand when you can afford to do so.
Wait six months and make the same offer again
The longer a domain is on the market, the more open a seller may be to lower offers.