tonnesoffun.com
How much do you think it's worth, hmm?

I'd say 15-30 dollars at the most. The letters are too repetitive.

I'd appraise it as worthless.

It's hard to spell and remember, so I think a few hundred USD if you have luck and find someone willing to pay. Otherwise as McJ stated, you may hardly find a buyer.

Breakup : tonnes of fun

As said earlier, it's a bit hard to remember.
If "tons" was used instead of "tonnes" it would have been much better.

- Long domain name
- Not easy to remember
- But it's a .com

So, i'd say low/medium $xx [undeveloped].

Tonnes is a metric ton.
1 ton = 2200 lbs
1 tonne = 1000 kg

and i think 2220 lbs would be 1100 kg.

Max, incorrect.
tonne - Google Search

ton - Google Search

Check those two links.
1 kg = ~2.2lbs.

1 tonne = 1000 kg
1 ton = ~2200 lbs