What should it be? The recommendation I make is buy big ads and pay for colour. When buying a newspaper ad, deciding between greyscale and colour may be a difficult decision to make if the newspaper is adding a hefty colour charge to your ad cost. You can see with this example which one packs the most punch.
If you want to go with the cheapest price in this case, I would say your resulting ad will not be as successful as the colour version.
Then again, there are ways to explore making this work for you in black and white in another way, by converting the photograph to art shapes. This will depend on the product or image you have and what your message will be.
Sometimes simple is good if you have an image easily translated to art shapes.
Maybe your neighbourhood coffee shop is trying to get you to their new location. Here’s a very basic example using a photo taken of the outside of the business so you can recognize the shop on the street. Start with the converted photo, add the logo and message, and you see it is a clean attractive way to go and you save the cost of the colour charges!
Let your designer guide you – they will be looking at you and/or your product with fresh eyes and their job is to portray your materials in the most eye-catching way possible. Maybe the ad above could have been a photograph of logo’d coffee cup instead – still having the address big enough to draw the eye at the end of the ad; so you need to be willing to explore options that could be more interesting than you originally thought about.