"unusually expensive" is correct and usable in written English.
You can use it to describe something that is more expensive than expected. For example, "The new phone model was unusually expensive, coming in at $900."
A12 Unions to Pressure Wal-Mart The A.F.L.-C.I.O. and more than a half-dozen unions are planning an unusual -- and unusually expensive -- campaign intended to pressure Wal-Mart to improve its benefits.
Was someone at the commission keeping a thrifty eye on expenses, and did they work out that Brussels seemed like an unusually expensive place to hire movers?
The HPV vaccine is unusually expensive.
Why this was were models in Sid unusually expensive?—Sumanovic never revealed.
And it is unusually expensive to maintain because it is so long, having been built at the Hudson's widest point.
Mr. Fiddle said older buildings are getting unusually expensive makeovers near ground zero, the Brooklyn Bridge and Madison Square Park.
The studio acceded, and in April he began an unusually extensive (and expensive) eighteen-day reshoot.
— Democratic voters in Virginia selected a state Senator as their party's nominee for Governor Tuesday, defeating the close Clinton confidante Terry McAuliffe in an unusually contentious, expensive and closely watched primary for governor.
The three buildings benefited from parcels that Astor acquired for extra light and air — an unusually enlightened if expensive touch.
He noted that a third of all cancers are directly attributable to smoking, and that many smoking-related cancers are unusually deadly and expensive to treat.
It was an unusually aggressive and expensive campaign for a debt-ridden company stuck in a defensive mode in recent years.
