DBA registration varies widely from state to state, and likely county to county in some states (don't know about foreign..)
This is what the Secretary of State office says in Minnesota:
"The filing fee for original assumed names and amendments is $25. The original filing is good for ten years, after which it may be renewed. Original filings and amendments that show changes in the original information must be published in a legal newspaper qualified for the county of the business' location for two successive issues after the filing is made with this Office."
There are of course benefits to being an LLC (or an LLP) and there are benefits to being a DBA as well...
For tax purposes, keep in mind.. you're able to claim a loss two out of five years you're "in business" without raising too many eyebrows at the IRS. Which means, within your first couple years with the DBA, get as many of the toys as you think you'll need, and write 'em all off, likely you'll end up paying little or no taxes that year (to compensate for the LOSS of doing business in the film biz)
Granted, you can do this EVERY year, just be sure you keep all your receipts.
If you go the LLC or LLP (or heaven forbid full blown corporation) route,
HIRE AN ACCOUNTANT!
As for paperwork, I highly recommend
http://www.amazon.com/exec/obidos/ASIN/0240804198/intensitymedi-20/102-0169606-6562523?%5Fencoding=UTF8&camp=1789&link%5Fcode=xm2]"The Complete Film Production Handbook - Third Edition"[/i] by Eve Light Honthaner ... it's about $35, book & CD combo, chock full of forms galore.