Almost a bump. Looks like these can sell for 20 keys (see most recent comments on first link). Looks like these can get 14 key offers fairly quickly, with a bit of patience you can get up to 20. Just going to expand the range, price is 5 months old.

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http://www.tf2outpost.com/trade/7585089

B/O: 20 keys

Sold: Yes (collecting)

Price:

- 20 keys

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http://www.tf2outpost.com/trade/7472837

B/O: None Set

Sold: No

Offers:

- 15 keys in items

Offer value: 14 keys

Best offer: 14 keys

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Conclusion: Personally, I wouldn't pay 20 keys for it, but someone is obviously willing to, and we can't ignore that. 14 keys is a good low end, as most price checks are around two Bill's and that's the range off most offers. These are the only two on OP at the moment.