I don't know anything about the seller you are dealing with but the gas tank on your bike now can be repaired with Caswell Gas Tank Sealer for around $50.oo or so.
You could buy a new over size after market tank for around $90 to $120 according to the graphics you chose

 

I'd ask the shop why they think you need a new tank. If it isn't leaking, there's no need to replace it IMO, and even if it has some pinholes, Caswell epoxy tank sealer will fix that. Caswell is easy for a do it yourselfer to install. No shop needed. If they can't come up with a good answer, I wouldn't do any more business with them. Is this an independent shop or a stealership?

Many shops charge around $45 to sonically clean a carb. I they aren't in that ballpark, that's another strike against them. I know time to work on bikes can be hard to find, but when you do it yourself, you know how well it was done. Almost anyone with some mechanical aptitude, a good service manual, and the helpful folks on this forum, can fix about anything on a Rebel.

 

I went to Extreme Powersports. I'd heard a lot of good and bad things about them. Their price for labor is definitely ridiculous. I talked to them about the tank and they said that it was just horribly rusted, so they think I need a new one. I will say, when I took a look myself, it was so rusted I was surprised there weren't holes in it. Is there a good way to tell if it is too far gone for Caswell to work (or be worth it)?

I'm considering getting a new tank (conventional size or oversized, not sure yet) just to be done with it and know it's good to go. If I go that route, is there a decent brand or seller to go through? And a place to find them?

 

$450 is aout what Honda wants for a new factory gas tank.
Done right the Caswell will adhere well to the roughness os the rusty tank.
the tank is too far gone if there is a hole roughly the size of a quarter in it.
piholes that weep a little uel will be sealed by the Caswell repair.I used it on my wife's Virago 920 and salvaged her tank

 

I have used Caswell on many tanks that should have been thrown away and each tank turned out great! I put an over size tank on a 250 and a 450 just because I like the look of the larger tank, this is a link to the tank I used (unfortunately I see they are out of stock, I have not contacted them to see when they might have more)
Honda Rebel CMX 250 CMX250 Gas Fuel Tank Black - JMEI

 

Ok, I am a lot more confident now about keeping the fuel tank it has. Thank you, everyone, for all the feedback and advice. Once I get my Rebel back from EPS, I'm going to seal the tank myself. If it doesn't work and I do need a new tank, I'm only out about $50 and some time. I'd much rather handle these things myself from now on.

Once I try my hand with the seal, I'll post an update. Hopefully it'll be good news! Thanks again.

And thank you for the link, Old School. That tank might have been out of stock but there were plenty of other similar products that I was interested in taking a look at. Nice to know I don't have to resort to something like ebay every single time I'm looking for a mad or upgrade.

 

There is enough material in the Caswell kit to coat two standard Rebel tanks. You can hang onto the extra, or possibly sell the excess here.