Embassy is walking distance but not cheap unless you have a full room. Free breakfast and happy hour will bring down  the price per person.

Other motels will require a drive. What else are you doing in Des Moines? Get somewhere close to that. Don't rule out Altoona or Ankeny.

Unpublished rates at Expedia or Hotwire in the downtown area will  get you the Quality or Holiday/Mercy.

Funny story. We stayed at the Embassy with the grandkids. They had heard about Blazing Saddles wanted to see it. As we were taking pictures in front of it, I heard approaching sirens. 

"Time to go boys" 

"Afraid of the cops grandpa?"

"Think about it - middle aged man, 4 teenage boys, gay bar..."