Hyphen tolerant buyers can hand reg a similar double hyphen name: online--forextrading.com
Here's a crude value model. Consider the similar pattern "credit card application" dot com $235 sale as a benchmark, then adjust "online forex trading" up or down based on keyword search and cpc strength.
adwords keyword exact match latest update: "online forex trading" 49,500 / $9.74; "credit card application" 301,000 / $12.48
"online forex trading" has 16% search strength and 78% cpc strength of "credit card application".
(.16) x (.78) = 12% combined relative strength
(.12) x ($235) = $28 relative value*
* could be shaved further to account for market deflation since 2007