Old Bike / New Bike

My Norco 21 speed bike is 15 years old now. I had taken it out for a ride (albeit a 1/2 block only) a while back and noticed that the nut had mysterious dissappeared off the back wheel, which holds the tire in place ! I was very lucky I didn't get into a serious wipe out. Which just goes to prove one thing - MAKE SURE TO CHECK ALL SCREWS,BOLTS,& BRAKES ETC. BEFORE YOU GO ON YOUR RIDE. Well that halted all bike rides for a while. One would think it would be easy to find another nut for the wheel pin, but noooo it has been a big challenge. I have been to several bike stores and the nuts they give me don't thread onto the pin or bolt (whatever that thing is called)it just stops threading. Seems like the pin might have to be replaced. That whole thing might cost me 35.00 to fix it, plus 60.00 for a tune up, plus new cables and inner tubes for a grand total of over 100.00 dollars. Which begs the question, do I just put that money towards a brand new bike and forget the tune up ? I don't need a super expensive fancy mountain bike, just something to commute to work on. I am really thinking that I might just put it toward a new bike.