a balloon arch is an easy decoration to make with no cost beyond the helium, balloons and fishing line:

---using fishing line (the thicker the better), tie one end to a fence...this is your starting point
---inflate two balloons of the same color with helium and tie them together...do not tie them individually first
---take your two balloon creation and wrap it twice around the fishing line approx 3 feet from the fence at the knot so that they are opposite from eachother on the line...this will keep the balloons in place on the line...you can also wrap the line with tape if you feel the need to additionally secure the balloons and prevent them from slipping off the ened (i've never had this happen)
---take another two balloons of a different color (or the same if you want a one color arch) and repeat the process with the balloons being across the line from eachother and fitting in to the gap left by the previous two balloons
---repeat over and over again

this is very cost effective and still looks very nice