Not sure what your idea of cheap is but my idea of cheap is the following:

I would recommend the HP DL580 G7 with the PCIe expansion board installed. Make sure you get it with the PCIe expansion board since that adds six more PCIe slots two of which are x16 slots. Two x16 slots are on the main board. I was lucky enough to purchase one for $330 and the other four at $487.

One of these systems currently has four double slot sized GPUs installed inside. These systems can take full size GPUs.

Purchase the least expensive HP DL580 G7 and replace the processors with four Xeon E7-8837's ($44 on eBay). With these processors you can get about 1560 H/s mining Monero on the CPUs alone.

To power the GPUs you will need some of these cables as the power connectors in the DL580 G7 are proprietary.

See this post: https://bitcointalk.org/index.php?topic=1955358.msg33071070#msg33071070

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I recommend that you go back to page 31 and start reading from there as much more details about the HP DL580 G7 are provided.

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EDIT: 6/2/2018

Be sure to first update the BIOS to the latest version before upgrading the processors to the E7-8837's

Here is the latest BIOS for the DL580 G7 2018.02.22(A)(22 Mar 2018)
https://support.hpe.com/hpsc/swd/public/detail?sp4ts.oid=4142793&swItemId=MTX_0b8f3842a4cd4835b6dd193ee9&swEnvOid=4184