Actually The UMP is not a lower cost alternative as much as it is a replacement to the mp5.
The Ump is designed off the g36 gas system, which is more efficient. The mp5 is based off the g3 system, This system is old and outdated, and technology has advanced since then.
The polymer holds up better than steel . The weapon wont freeze up in cold weather.
You can get which ever Caliber you want. Be it 9mm, 40 cal, or .45 Cal (i think theres even a 10 mm) The Mp5 only comes in 9mm, There are some 40 cal and 10 mm versions, however these were special order options, i think they only went to some LE organizations in Europe.
A new addition is last round hold open. Which means you can stick a mag in and slap the release and your ready to go, and MP5 doesnt have this option (they were a few, but that was a special order for some state side Govt agency i believe). On the Mp5, every time you empty a mag, you have to open the bolt, stick in a new mag, and slap the knob to close the bolt.
Also, the Mp5 is a very costly gun, Adding a RAS system from KAC to this weapon after buying it is costly. However buying rails for the UMP forgrip would only run you $50 - 100, I think the RAS system is around $300 - 400, maybe even $500. And add acessories on top of that and you get a High price weapon.
Is the UMP cheaper than the Mp5? Yes, Is the UMP worse than the Mp5? No.