I just repaired one today for a customer of mine and it was a nighmare.
He had about 10 cracks in his fiberglass tub!!!!!!!! He told me 2 on the phone. He also failed to tell me the dang thing had been repained before and NOT professionally. It was one big chip I had to sand out. I made the mistake of believing his description and quoting him over the phone. I only charged him $225 and I put in about 10 hours, filled ALL the imperfections with bondo, glazing comp, epoxies, adhesive foams, AND ALSO reglazed the tub white on top of it all. It was perfect, he ripped me off but oh well he was very happy. I should have gotten about $450. Even that woulda been a bargain compared to the 2 grand a plumber drywall and tile guy would cost to change it out.
If he coulda done it himself with the home depot 100 dollar tub, and knew how to do it, and we were certain the drain alighned in the concrete foundation, I would have recommended it in a second, and really I tried to hint it so I didnt have to do it. But he said he did not even want to spend time working on it, and he didnt want the bathrooom torn up a week while a new tub went in, he only had one BR so I guess thats logical. So I had the pleasure.
Use the foam. But first, drill a hole at the beginning and end of EVERY crack... its so the cracks will not get any longer. The holes stop it. And you ALSO want to use these holes to squirt your foam in, so check the bit size to the applicator. Get your foam in there, and hope ts not ultra thin or it might lift a little bit from the foam.
let Foam dry without weight on it. Now, clean off the mes from the dry foam, razor and acetone help, or sandpaper. Get a power grinder, gring a notch in all your cracks. Fill the notch, NOT all the way full, with 3500lb test fiberglass epoxy (3$ at the store). Let dry. Get some bondo and sandpaper, cover the notches ans holes, and sand away.
Your on your own with the finish fo paint it. You cant buy the stuff I use professionally. Home depot sells a bath paint kit that MIGHT last a year before chipping and look HALFWAY decent if your crafty. Your best option.
Thats how its done, it works, and I am a pro. Sometimes theres other things like HOLES etc that require a different process.
Options:
The cost for you to fix it yourself is under $100 and just lotsa time. it will look "OK." It will last a yr or two but fade well before 2.
If it was me it would be around $300-$350 and thats what I have gotten in about 10 other states when I get work abroad. It will last up to about 10-15 years. It will initially be shinier than a new porcelain tub. It will appear to simply be new to everyone. It will take half a day at most, unless foam is 8 he foam, and be useable in 24 hrs. warranties with our type of business tend to run 1-5 years, mines usually 5 but 2 if theres repairs like this.
An insert. This will be $2000. Dont waste your money. Its the same price as a tearout, your only saving on time and mess. It will get condensation between the insert and tub over time creating mold. They ARENT good in the long run.
Replace it.
a contractor will be up to 2500. DIY for a couple hundred, and maybe 4 days work. You may have to remove door jamb to get it out and a new one in. I would sawzall the old one out. Replace your drywall but use hardibacker or durarock instead of the greenboard and tile over it.