1) in-House engines well, but you need to keep a team of programmers, and to write functional web store a few hours will not work. But, as said previous person — depends on your needs.
We are now set to move shop just on the finished engine. Chose Magento, but do not know how much was the right choice, because from scratch it quite difficult to understand. But programmers familiar with this pull on design is not very complicated.
OSCommerce is better not to look — the old system, the code is written badly, quite a lot of bugs can come out.
On Habre it is possible to look for a review of ready-made solutions.\\
2) prices — from zero, if to take ready-made skins and ready to pull the engine, to the "how much."