Many Exchange Traded Funds are designed to track double or triple the return of an index or the reverse of an index. So what happens if that ETF goes negative? Does your brokerage firm send you a bill? This warrants further research on my part, maybe for a more extensive article.
If you are looking for really low priced ETF's, here are a few that trade for less than $5. Shown is the recent price.
ELEMENTS CS Global Warming ETN (GWO) $4.92
ELEMENTS Rogers Intl Commodity Energy ETN (RJN) $4.84
Ultra Financials ProShares (UYG) $3.84
Internet Infrastructure HOLDRs (IIH) $1.77
B2B Internet HOLDRs (BHH) $0.24
Author does not own any of the above at the time this was written.