Ok, I went to both the EB Games in Ringwood (one in Eastland and the other at Ringwood Square) to get some confirmation of trade-in prices for DS consoles, also to try and clear up any confusion that I had (and anyone else had).
Started off at Ringwood Square first, and they had an ad hanging from the ceiling advertising the 3DS for only
$249 with the trade-in of the DSi; so that works out that the trade-in value of a DSi towards a DS was $100. The sales assistant didn't seem too interested to serve my mate and I so we went to the EB Games in Eastland Ringwood.
I will confirm again that the trade-in prices for DS consoles towards the 3DS are as follows:
DS Lite -
$80DSi -
$140DSi XL -
$200I've got photographic proof of this:

Also, at EB Games Eastland, there was a similar ad hanging from the ceiling as the one I saw in EB Games Ringwood Square, though this ad was advertising that the 3DS could be purchased at
$209 with a trade-in of a DSi, a whopping $40 less than the same deal I had seen earlier at Ringwood Square!

I'm not really surprised that EB @ Swanston and Elizabeth Streets don't have the same promo. Back in December when certain EB stores had the Manager's Special promo of an extra 50% trade-in value towards games, some of us worked out that this wasn't nation-wide and the EB stores that had this promo had a games-specialist competitor in that same shopping mall (either a Game or a Game Traders).
I know I went to check both Melbourne CBD stores in December about that 50% extra trade-in promo and neither store had it. Dungeon Crawl is in the city, but it's not quite a "big" games specialist like Game.
Anyone else come across the "extra value" trade-in prices of DS consoles towards their 3DS consoles? Surely I can't have lucked out with my local EB Games being the only one offered this promo?