What should be my budget for Bing Ads in Digital Services niche?
Although I don't know about your product but for Bing consider about $2000-$4000 per month according to your needs.
$1000 minimum. In some cases Bing ads are even costlier than adwords.
If you are selling Digital Services then check how much your competitors are spending too. Sometime it happens that maybe you are selling at a low price but as the competitors has jacked up their promotion so you are getting much less sales than you must actually get.