You choose the price as ads on XING work on an auction basis, meaning that your ad competes with other ads aimed at the same target group at that given time.
When posting an ad you set a maximum bid (XING will recommend an amount). If your maximum bid is higher than competing ads, your ad will be displayed. You then do not pay your maximum bid, but the amount neccessary to overbid the second highest bid. You can also set a maximum budget and a daily budget for each ad, and you can pay per click or per thousand ad impressions (CPM). We'll only charge you for the clicks/ad impressions you receive up to your budget limit. You can check your balance in real time by going to Manage ads.
At the start of each month you'll receive an invoice for the actual number of clicks or ad impressions. Clicks on the ad itself and on the person who posted the ad (i.e. your profile or company profile) will be billed. If you don't receive any clicks or have any ad impressions, you won't be billed anything at all.