There are many things to take into account when setting the prices for sponsorships for your podcast. Typically, all that has been important is the number of downloads you have per episode, but there are many other factors that are just as important, if not more.
If your podcast is about something very specific, so then your audience might be smaller, but very segmented. This segmentation can mean that your audience may be very interesting for some advertisers and hence raise the price they are willing to pay for a sponsorship.
If your listeners are from mainly one country then this may influence the value as most advertisers are interested in targeting audiences in specific locations, with some exceptions.
Are you engaging with your audience in other ways than through your podcast? Do you have a Patreon with a following, or a Discord with a lot of engagement? Is your audience engaging with your content, both podcast and in other places? The more engaged your audience is with you and your content, the more interesting for a potential sponsor.
As you can see, there are many things to consider when deciding on a price, and it all depends on which sponsors are interested in your particular subject and audience. Therefore it is hard to give specific recommendations, but to give you an idea we will try below:
If your show has less than 1000 downloads per episode, then a flat fee of €15-€30 for pre & mid-roll ads is something most advertisers are happy with, but as mentioned, this can vary. For post-roll ads, the price is typically a lot lower, as many listeners will leave before it airs. Note: Check with the sponsor if they want to be exclusive (meaning you won´t advertise any other sponsors in the same episode).
If you are also offering custom types of sponsorships, like interviews, reviews, baked-in´s, etc., then you can charge more for these as they carry more work on your end as well as a higher ROI for the sponsor. Depending on the type of custom sponsorship you agree, a typical price would be from €40-€100.
If you have more than 1000 downloads, then you would typically adjust a CPM (cost per thousand listeners) to your expected downloads for the next episodes. The CPM would normally fall within €20-€40, so if you had 2000 downloads, the price would be €40-€80.
For more custom collaborations, the price can vary a lot, would normally fall within €40-€100 CPM.
Some sponsors may also give products/services for you to test, or CPA-based commissions as well based on certain actions by your audience, like buying a product or signing up to a service. This depends a lot on the type of sponsor.