You might be able to build a 6x AMD Rx 580 4GB rig for $2000 or a 6x Nvidia GTX 1070 rig for $3000. This would be based on USA sale prices... most other countries in the world will be more expensive.  Based solely on the prices of altcoins today I would find the Rx 580 to be a quicker ROI, but the GTX 1070 is more versatile (which bodes well for the future). AMD also requires more fiddling with to get optimal performance, which may be a negative if you are new to mining... on the other hand a $2k rig is allot cheaper then a $3k rig to start out with.