Ke... I think a few days in San Ignacio is well worth it - even with getting there and paying for a hotel room for one or two nights, you're bound to pay less than day trips from the island - and you get the added bonus of actually getting to spend some quality time in the area instead of just what you see driving through. I put together a little price comparison chart when I was getting ready for my trip and some of the (approximate) price difference I found were...

Barton Creek Cave Tour....
From AC is $180 US
From SI is $30 US

Mountain Pine Ridge
From AC is $185 US
From SI is $35 US

Xunantunich, Cahal Pech, El Pilar
From AC is $180 US
From SI is $40 US

Caracol Tour
From AC is $225 US
From SI is $95 US

Tikal Tour
From AC is $355 US
From SI is $100 US

Sooooo... espeically if you're looking to do a few tours, it is definitely more economical to do them from San Ignacio than the island. However, if time is more the issue than money, the day trips are definitely available.