What's a good IP strategy when the budget for patent is limited?

I'm not that familiar with all the "dos" and "don'ts" or "cans" or "cannots" of German law, but if you are able to file a provisional patent application in Germany, or are permitted to file a provisional application in the US even though your IP was developed in Germany, then I would proceed with a provisional patent application. Speaking for US patent procedures, you can file a provisional patent application for a relatively inexpensive amount of money provided that you can put together an enabling disclosure of the invention, and the filing fee is only $130 for a small entity.
This provisional patent application establishes a priority date which effectively establishes the fact that you were the first inventor for such technology despite others who may file patent applications subsequent to your provisional patent application. The one significant drawback of a provisional patent application is the fact that, in and of itself, it will never be examined and will never lead to the issuance of a patent unless you effectively perfect/convert the provisional application into a non-provisional or PCT application within one year of the filing of the provisional application - note that the one year deadline is a hard deadline, meaning there are no extensions of time permissible - if you fail to file your non-provisional or PCT application within the one year time frame, your provisional application, and your priority date is lost. So, during that one year, you can use such time to assess your financial situation, try to obtain funding, see what potential commercialization appears to be like, and so on.

You may wish to find out whether your university has a technology transfer or IP department which helps researchers commercialise their inventions, be it through funding or commercialisation expertise. Some universities are prepared to fund the cost of IP protection in return for part ownership of the patent rights.

1. There is a special public sponsorship program (WIPANO) which might be helpful for you (http://bmwi.de/DE/Themen/Technologie/Rahmenbedingungen/patente,did=774168.html).
2. You may file a kind of a provisional patent application as was suggested by Steven Weinrieb with the German Patent and Trademark Office, e.g. a patent application which establishes a date of priority. Within one year after the date of priority further regular patent applications may be filed claiming the priority of the earlier application and having the effective application date of the earlier application. Such a provisional application may be filed in any language with the German Patent and Trademarks without paying any official fee. Although the application will be deemed to be withdrawn unless you pay official fees you will get a date of priority.
3. It may be helpful to receive an examination report within the first twelve months after the date of priority in order to gain a valuable basis for deciding on further filings and for finding an outside investor. In this case it is advisable to file a regular application, i.e. with payment of official fees. The official filing fee is 40 EUR for online filing (incl. 10 claims) or 60 EUR for paper filing (incl. 10 claims), the examination fee is 350 EUR (if no search request has been filed). A search request (300 EUR) may be filed independently from filing an examination request. For first filings (i.e. filings which do not claim a priority) the German Patent and Trademark Office in most cases issues a search or examination report within 9 months after the application date when the search fee or the examination fee is paid at the filing date. This also applies for applications filed in English without the necessity of filing a German translation before the issuance of the search or examination report (nevertheless, a German translation has to be filed within 12 months after the date of priority in this case). Accordingly, filing a German patent application with a search request will give you a valuable basis for deciding on further filings at comparably low official fees (340 EUR for online filing incl. 10 claims).
4. Independent from whether the application is a “provisional” or regular application, it is advisable to entrust a patent attorney with the preparation of the application documents. Once an application has been filed, no further matter may be added to this application. When a subsequent application is filed within the first twelve months after the date of priority, further matter may be added to this subsequent application, but this may involve several risks. A patent attorney has much experience in how to appropriately draft a patent application and find the optimum strategy of how to protect your inventions.

Apart from the helpful suggestions in patent matters, it might is worth a thought to search for a future concept together with your university. As an example from our latest experiences in a comparable case:
The researchers (former students and professors) established a limited liability company ("GmbH") for the commercialisation of the technology as a spin-off ("Ausgründung") from the university. The University on the other hand agreed to file the first patent applications (Germany and EU) on its own cost (and to its own benefit). In the next step the university contributed the IP as a non-cash contribution ("Sacheinlage") in change for shares in the company. Then an outside Investor was found, the worldwide applications could be filed and the company could thrive and prosper.