We had a good refinishing experience on an old cast iron tub. We paid about $300 in 2007 and about $40 for a new drain. It lasted until 2015. If you go that route definitely check Angie's List reviews, as someone can do a crap job and it won't last. FIRE me is also right that you can't use anything abrasive on the finish or you will ruin it. Honestly, the reason so many finishes are in crap shape to begin with is because someone used Brillo and/or Comet on it at some point in time. This will ruin any porcelain finish and make it damned hard to clean. I won't even use baking soda on my brand new cast iron tub in another bathroom.
Now that your finish is already ruined, you can probably google how to use sandpaper or something to clean it and even it out. I recall watching a home show in which someone did this, but that might have just been to sell the house, because I can't imagine it didn't get dirty again if they didn't reglaze it afterwards.
There's also the Re-Bath option.
How much remodeling are you doing? Could you post pictures? We are about to take out the refinished 50+ year old tub and put in a brand new 60X30 cast iron Kohler Villager that I only paid $350 for. The reason people don't just replace tubs all the tub is because they are a royal pain in the ass. The main reason we are doing it is because #1 the tub isn't level and #2 the bathroom is being gutted down to the joists. For that reason, it made sense to go with a new tub.