How about we try to establish if 20,000 Republic Dataries is a "small" or "large" amount? If one's to compare those 20k rep credits with presumably Imperial Credits in ANH, "Ten thousand? We could almost buy our own ship for that!"
and a little later: "HAN: Seventeen thousand! Those guys must really be desperate. This could really save my neck. Get back to the ship and get her ready."
In other words, it seems like a lot. Qui-Gon seemed confident that 20,000 would cover a new hyperdrive, although one could argue about how much a Jedi Master know of shopping.
Mind you that this is purely subjective, but it seems to me as if 20k is "a lot", which would normally pay for a new hyperdrive or even a small ship without question.
Some script quotes to freshen memory:
WATTO : ...Here it is...a T-14 hyperdrive generator!! Thee in luck, I'm the only one hereabouts who has one...but thee might as well buy a new ship. It would be cheaper, I think...Sying of which, how's thee going to pay for all this?
QUI-GON : I have 20,000 Republic dataries.
WATTO : Republic credits?!? Republic credits are no good out here. I need
something more real...
QUI-GON : I don't have anything else. (raising his hand) But credits will do
fine.
WATTO : No they won'ta. QUI-GON, using his mind power, waves his hand again.
QUI-GON : Credits will do fine.
WATTO : No, they won'ta. What you think you're some kinda Jedi, waving your hand around like that? I'm a Toydarian. Mind tricks don'ta work on me-only money. No money, no parts! No deal! And no one else has a T-14 hyperdrive, I promise you that.
QUI-GON : ...Obi-Wan, you're sure there isn't anything of value left on board?
OBI-WAN : (V.O) A few containers of supplies, the Queen's wardrobe, maybe. Not enough for you to barter with. Not in the ammounts you're talking about.
QUI-GON : All right. Another solution will present itself. I'll check back.
QUI-GON puts his comlink away and starts out into the main street. JAR JAR
grabs his arm.
JAR JAR : Noah gain...da beings hereabouts cawazy. Wesa be robbed un
crunched.
QUI-GON : Not likely. We have nothing of value, that's our problem.
It does seem to me as if Republic Credits are worthless to Wattoo, not just unstable or of low value, in which case he could ask for a premium. The T-14 hyperdrive appears to be very expensive as well "thee might as well buy a new ship".
"Republic credits?!? Republic credits are no good out here. I need
something more real..."
I think this is the key statement. For whatever reason, credits are worthless to him. As some have speculated already, "more real" could mean physical coins as opposed to a credit chip, which no doubt is what Qui-Gon had. But without more details, we'll never get past speculation and into knowledge.