Fairly close to "LaSalle in the business district", there is a long stretch of Wells from Polk South to Roosevelt that is unzoned and unmetered on the EAST side of the street. This generally fills up during workdays as savvy parkers have found it, but if you're looking for a place in the early evening as people begin to leave or on the weekend you should be OK in terms of finding a space.
There are also several surface lots in this area that charge between $7.50 and $8.00 per day, and I'll bet you could swing a deal if you chatted them up a bit. Try on Clark south of Harrison and then the lots on Polk West of Clark. The lots on Clark Street have the advantage of being only a block or so from the LaSalle Blue Line stop, which takes one right to O'Hare.