Amazon’s efforts to take on iTunes have been boosted by an undisclosed investment in Amie Street, a company that runs a website that sells music at varying prices based on demand.

Currently, artists upload their songs to Amie Street and the first few customers can download the song for free. As demand grows, the song's price rises, capping at $0.98 (£0.50).

The songs on Amie Street come without DRM (digital rights management) software, a concept that appears to like. When announced in May that it would open an online digital music store later this year, it said that the songs would be DRM-free.

Amie Street aims to combine online social networking with music sales. When users recommend songs to friends, they receive credit for buying more songs on the site. The more popular the song gets, the more credit that user receives. Users can also post reviews and recommendations about songs on the site.

Amie Street shares 70 percent of the sale of each song with the artist but it keeps the first $5 in sales to cover its own costs.

The concept of varying prices for songs is contrary to the model in iTunes, the most popular online music retailer. Songs in Apple's iTunes cost $0.99, except for DRM-free songs, which cost $1.29. That has been at odds with some record companies, which wish to use a varied pricing model. Apple, however, has held firm to its pricing plan.

When announced that it would launch a store, it said that it would sell songs from EMI Music and thousands of other labels. The DRM-free concept may prevent the online retailer from scoring deals with all the major record companies, some of which have yet to sell their music without the software.