Zoey, for clean, close and cheap I would have to recommend the Alpine Inn on Katella. DH and I lived there for a little while, and we were quite pleasantly surprised by how clean the rooms were! Their rooms were recently remodeled; I believe they changed management as well. The bathrooms were very small (but sparkling and spotless) and the decor was bright and pleasant. We were able to get a King room for 2 adults in the middle of summer for only $42 per night! They have a nice pool, and a continental breakfast, though I wouldn't recommend the breakfast because it's only pre-packaged danishes and one pot of coffee.
The reason I also recommend the Alpine Inn is because they are just steps from the Convention Center crosswalk, a footpath that leads directly into the Timon parking lot. From there, you can take the parking tram the rest of the way into the main entance plaza. It's about the same walk as you would encounter from any of the closest hotels on Harbor, with a short tram ride tacked on. Absolutely worth it.
PLUS, if you request a room on the 2nd floor facing DCA, you can watch the Maliboomer shoot up all day long, and even watch California Screamin'!
We had a room on the first floor (I want to say 202, but don't quote me on that?) and even from our window we could see the top 1/3rd of the Maliboomer. That was really special, because I was working at the Maliboomer at the time!
As it is an economy motel, you do forego some of the extras like free local phone calls, etc. Local phone calls cost 35 cents, and even the ice machine requires change I think. It's up to you whether or not you would use those amenities enough to justify the extra expense somewhere else!
For one step up in price and quality, the Desert Palm Inn & Suites next door is one of my absolute favorite places to stay. DH and I honeymooned there! We had a King suite with a bedroom & living area w/ sofa bed for only $65/night in May!
Sorry...to blab your ears off. Good luck making your choice!