As to the question of why, those who chimed in seem to be all correct.
Quick game design 101:
In game design, there is a concept called inflation. As players advance in the game, their means of accumulating currency increase. In Hay Day, the prices of the buildings go up because of this. But with decos, we prefer to keep them all relatively affordable, regardless of in which level they unlock. But they still need to be able to remove significant amounts of currency from the economy, which is why they increase in price per unit purchased. So initially they might be a breeze, but as you accumulate them (if you so wish) they become more effective in balancing out the surplus in the economy.
You may read more about Inflation here:
http://gamebalanceconcepts.wordpress.com/ (Do a search for "inflation", as there's lots of other things in there). It's also a good resource for learning more about game balance in general, if you're into that. :)
Cheers,
Cami