I would think if you got the dates right the 30-40 entries Steve is looking at would not be too much of a problem, especially with the prize payout you are suggesting.
Barossa struggled for entries over the last few years but still normally ended up with around the 40 mark with 1st prize around $1000 - $1200.
Would be great to see another decent tournament back on the SA calendar, with the prize money suggested would think a few interstate bowlers would look at travelling to the Riverland for weekend
Reckon I know 2 people from up your way George would come, although I think they may be heading overseas the dates you are suggesting
Hope it all comes up trumps
Cheers
Colin