Price for posts on my site?

So I recently got an email from another site to guest post on my site and they brought up cost.
Since this is the first time one of my sites has had interest from another company to post on my site, this is new to me.

The site metrics DA: 20+ PA: 30+ niche is games and technology

What would be an acceptable cost for a post on my site?

There are no hard set rules here. As links are kind of associations and up votes you can decide how much your word worth for you. If you link out to cheap sites your site will be associated with cheap sites that (in extreme cases) could hurt your ranking. So you are running some risks here. You decide how much does that risk worth to you then you may ask their budget and exactly why kind of post do they want.
Link in an old post
Brand new post.
Unique post or it's just a syndicated article.
If apply: ********, nofollow, product links?
Do they expect visitors or just for building links?

Email other sites in your niche if they are offering guest posts. Once they reply you with the price, you can decide the cost for your blog. Keep it a few bucks cheaper if your domain metrics are weaker than the websites in your niche.