(Undated)  --  Portland is ranked has having among the highest rental prices in the nation.  Online real estate investment management firm HomeUnion says Portland had the eleventh-highest rental market in the U.S., seeing an increase of three-point-eight percent over the past year.  The average single-family rental in Portland goes for more than 28-hundred dollars per month.  HomeUnion ranks Seattle the nation's most expensive rental market, with the average unit going for nearly 23-hundred dollars, a rise in the last year of nearly seven percent.