I don't really take mine in for maintenance. I am like you about keeping my machines cleaned and oiled. I clean and oil the bobbin case every single time I change a bobbin. After every project or approximately 8 hours of sewing I completely clean my machine of lint and oil it completely if it is an older machine. I've never had any problems with my machines. I also NEVER sew over pins so the machines do not get out of time. Now...I have gone over heavy seams and let the needle plate slip away from the leveling pad and the needle has hit the foot and busted. This always leaves a nick in the foot but emory paper smoothes out these nicks and then I check the timing carefully to ensure it hasn't knocked the machine out of timing. I am a bad example to other sewers but unlike my car that I am fanatical about taking in for regular oil changes and scheduled maintenance I never take in my machines unless there is a problem which is extremely rare. I believe that one time I took one machine in for maintenance because I moved and it was stuck in transit and storage for three months so just to be safe I took it in for maintenance. It was my Bernina 1630, an electronic machine and I paid $60 but that was probably 6 or 7 years ago. I wonder why the cost was so high for your machines? Was there quite a bit of build-up of old oil? Did it need parts replaced? I just purchased an old Singer 301 off ebay and the first thing I did was take it apart, clean it and oil it completely. I took apart the tension knob and cleaned it and put it back together to make sure nothing was broken. It runs like a charm! The older the machines, the easier it is to clean and maintain them, in my humble opinion! Also...never, never, never use anything but high-quality sewing machine oil. Other oils will thicken up and clog your machine. My ex once told me I should use some oil out of his garage..something like D&1 or other multi-purpose oil that did show a picture of a sewing machine...but I called the local sewing machine dealership and he said NO! Never! I hope you get a lot of worry-free miles on those machines!
Vanessa in Oklahoma