depends on how you are having them printed or if there will be a mix of color and black and white.
Hopefully a lesson in Spot color vs 4-color printing is in your future.
If you have a color photo anywhere in the design, you will be doing a 4-color print, whether it is a postcard, brochure or t-shirt (I added the t-shirt for a reason)
Using a lot of photos doesn’t matter. The price of the page will be the same whether there is one small photo in the very bottom corner or a full page collage of photos.
Where it might matter is if you are doing a press run with 4-color images and spot colors. Spot colors are mixed inks and require an addiitonal press plate on top of the 4 color plates used for the photos. Each plate is an additional charge. Or you could convert your spot color to CMYK to avoid that, but you should know how to do that so you don’t get a very noticeable shift in color.
As for that t-shirt, most places that print those use spot colors only. Not all of them will do a 4-color photo. So using that black and white image will be to your advantage there. There are some direct-to-garment printers out there and some CMYK-capable screen printers.
Alway always always read the printer specifications. And ask questions if you are unsure what they mean. Always better to ask than to have to pay for a reprint out of your own pocket.