I love staying in Chicago, but don't like to pay a lot.  My favorite place to stay is Club Quarters which is on Wacker and Michigan.  Generally you can find rates for under $100, but it all depends on the days.  Their rooms are small and more for the business traveler.  Don't expect a lot, but it's clean and in a decent location.  I'm not sure where the Marriott is, but it might be a little walk down Michigan.

Club Quarters Chicago

Lizette,  I love The Whitehall Hotel.  It is very reasonable and the staff is wonderful.  it is on Delaware across from the 4 Seasons.  i used to to the Four Seasons Hotel Chicago for High Tea and then back to my room at the Whitehall.  it is a fifteen minute walk on the Magnificent Mile between the Whitehall and  Mariott.

Try the Aloft Chicago City Center. It's only a couple of blocks away. I stayed there in this past summer and liked it a lot.

Lizette - I'd recommend checking Airbnb as you can often get an entire apartment for less than the cost of a hotel.  If you're also more social, you could book a private room in a shared apartment as well will help in saving $$.   Overall, that area can be quite expensive for hotels, but booking through Airbnb will be a fraction of the cost :)   Let me know if you have more specific questions about it and I'll be happy to help!

I would check Groupon or Living Social. They always have deals on hotels near there. Make sure to check the fine print if you do find something and make sure the dates work out.

Lizette - You should try a new service called http://www.tripbam.com/HOME.aspx  Tripbam allows you to set prices and area and they automatically have hotels bid against each other for your business. 

I've had lots of friends stay in Chicago for not that much by bidding on Priceline or booking on Hotwire.  It's probably still a little early for really great rates, I'm not seeing any large conventions on the calendar.  www.betterbidding.com might be useful if you decide to try it.

Chicago Getaway Hostel might be another option, it's located in Lincoln Park so it'd be a little bit of a commute in the morning but it's a nice safe area.  Holiday Jones is another hostel type option, it's located in Wicker Park so once again you'd have a commute but Wicker Park is a fun young area with a lot of interesting restaurants.

Keep an eye on Travelzoo, they are a deal consolidator and I see rates under $100 per night on there which isn't bad for downtown Chicago.

Try using Airbnb.

Consider using a website like Airbnb.com.  It's not the same as a hotel but if you're anything like me, that's a plus.  It can be a great way to save some money.

Hotels around the Mile are crazy expensive.  I prefer AirBnB.com to rent a condo or room for far less.