You have come to the right place.   We have all been exactly where you are.  The only thing you can do is gather all the info you can and make a decision.  Unfortunately you can never know if it is right or wrong, if there even is a wrong.  If you put him to sleep you will always wonder if you had to, and if you amputate you will always wonder if it would've been easier to put him to sleep.  It seems like the smaller dogs do pretty well with amputation and are up and running quite quickly afterward.  The post-op process takes generally 2 weeks and is very very hard.  If you decide to do it, you will question your decision.  Most everyone on here doesn't regret their decision to amputate though.  There are the other options of radiation therapy, which is used to relieve the pain but not get rid of the cancer, and limb salvage.  Radiation therapy doesn't work for 1/3 of those who get it and is expensive, and salvaging the limb is rarely an option.  Amputation cost will depend on where you get it---it will be cheapest at your vet probably, but you never know how many surgeries like that they have done and how good their equipment and anesthesia monitoring, etc is.  I would look around for a specialist because at the very least you can get a second opinion, which for me was a lifesaver because my vet was horribly ignorant and lied to me about some things. 

Cost can be a big issue.  It doesn't make you a bad person to question these things, because it can be expensive.  Tootsweet is smaller which means usually a cheaper surgery.  My dog Mac was 67 pounds at the time of surgery and these were the quotes for amputation: from the vet, 900...from the specialist 2300...from the cancer center, 3500.  We went with the specialist 2 hours away, and with extra diagnostic tests it came to 3,000 total.  Chemotherapy can put you back, but there is also the holistic route that is less expensive.  That includes things like diet, supplements, acupuncture, and so on. 

There is a lot to consider, and you're right this all really sucks.  I had to go through several doctors and tests and scheduled amputation appointments before Mac finally had his on August 10.  They thought he had osteosarcoma like your pup, but unfortunately the biopsy of the leg afterward confirmed that it was instead hemangiosarcoma which is a lot worse and more aggressive.  The tumor triggered kidney problems, and 5 weeks later he still is nauseated and doesn't eat like he used to and doesn't play.  But he moves around some.  HS makes them feel worse than OS, so who knows how he would have recovered if he had what your dog has.  In my opinion Tootsweet has a good chance of doing just fine on 3 legs.  But if you choose not to that's ok too.  Don't feel pressured by anybody to go either way, but know that whatever you do will be an informed decision made out of love so it'll be right.

Sending you virtual hugs and a box of those really soft aloe-infused tissues to wipe your tears