The cheapest option for data roaming in the EU (N.B. Switzerland is not part of the EU) with a German prepaid SIM card comes from Aldi who offer the "EU Internet-Paket 60" for € 4.99 which includes 60MB valid for 7 days. So you would pay
$0.10/MB and each time you have used up one of these 60MB packs you would need to order another one by a phone call to their computer-based customer service, which is neither really cheap nor covenient if you're looking for heaver data usage.
Actually I don't know of any cheaper offer from any other European operator even not for contract customers, so your requirement of availability in Germany doesn't do any harm.
While you are in Germany data usage with an Aldi SIM is cheaper as you can subscribe to one of the following options:
- 500 MB for € 6.99 (Internet-Flatrate M)
- 1500MB for € 9.99 (Internet-Flatrate L)
- 5000MB for € 14.99 (Internet-Flatrate XL)
But be warned that Aldi is based on the eplus network which often delivers subpar performance with bandwidth sometimes dropping to significantly below 1 MBit/s (this only applies while you are in Germany and using the SIM card's home network).
As you seem to look for heavier data usage there's no alternative to buying local SIM cards in each of the countries you travel to.
USB modems such as the Huawei E173 are often (but not always) sold bundled with O2 prepaid SIM cards by the large electronics retail chains (Saturn and Mediamarkt) for some € 10 and unlocking the ones produces by Huawei (see backside of box for manufacturer) is pretty straightforward. You just need to obtain a free unlock code from
here and enter it once the dashboard software asks for it which will happen once you insert another SIM card.
Currently a German online retailer sells the Huawei E173 for € 9 including shipping within Germany:
O2 Prepaid Surfstick Huawei E 173 weiss inkl. 2 Farbschalen o2Go
The Huawei E173 is a pretty good offer as it supports voice calls from your computer so you could order those 60MB packs directly from your computer instead of putting the Aldi SIM into a phone each time you need another pack.
As of router compatibility this depends on the router you intend to use, but usually Huawei sticks are the primary ones to be supported. You might also consider picking up a MiFi such like the American version of the ZTE MF61 which is named "Sonic 4G Mobile HotSpot" by T-Mobile USA and is sold unlocked for some $80 on eBay.com (from an American seller):
T-Mobile 4G Wifi Hotspot UNLOCKED GSM ZTE MF61 US Seller 610214625892 | eBay
It supports T-Mobile's American 3G frequency (1700/2100 MHz aka UMTS band IV), one of AT&T's 3G frequencies (850 MHz aka UMTS band V) and the major European 3G frequency (2100 MHz aka UMTS band I), so it would be useful for your trip but also back in the States.