I researched different sources of stock images.  My main research topic was price.  Note, pricing was current in June 2018.

UnSplash

  • free

Pixabay

  • free

Pexels

  • free

iStockPhoto

  • Two pricing plans
    • Credits that start out at $11 per credit and reduce with volume
    • Subscription plans
      • 10 images per month; access all images for $99 a month or $70 a month per year
      • 10 images per month; essential images only for $40 a month or $29 a month per year
  • Basically, images start costing 1 credit.

Shutterstock

  • Two pricing plans
    • Single project
      • 2 images for $29
      • 5 images for $49
    • Ongoing projects
      • Month to month is 10 images for $49 per month
      • Annual is 10 images for $29 per month

Adobe Stock

  • Two pricing terms
    • Month to month
      • 3 assets a month for $29.99; additional assets at $9.99 each
    • Annual
      • 10 assets a month for $29.99; additional assets at $2.99 each

Fotolia

  • Owned by Adobe Stock
  • Multiple options
    • On demand
      • $14 for 10 credits
    • Monthly pack
      • $25 a month for 5 downloads
    • Daily subscription
      • 25 downloads a day
  • Credits depend on the size of the image

Resources: