You don't have to buy anything, what's listed below is completely free or open source software.
1. Download
oggdec v1.9.2.
2. Download
LAME 3.97b2, the package that contains lame.exe. Note: It's the best mp3 encoder!.
3. Download
ALL2LAME.
4. Unzip ALL2LAME into it's own folder such as C:\ALL2LAME.
5. Unzip oggdec v1.9.2 into the C:\ALL2LAME folder.
6. Go
here to find out how to encode at your preferred bitrate/quality. Input that bitrate/quality you want into the ALL2LAME switches area.
7. Add your ogg's into the ALL2LAME window and click Encode.
And since you'll be transcoding ("converting") your audio files I highly recommend that you keep the original .ogg audio files, hence transcoding from one lossy format to another one will cause a loss of quality.
If you need any specific information or help configuring ALL2LAME just ask here and I'll help you.