Yeah, maybe a terminology issue, but what you want are cheap stays because even Breakfast in B&B in Italy are pretty meager. I much prefer a caffe and croissant to anything found in a hotel. Going in April is good, because you should still get off-season or shoulder season prices, except if you bump into Easter. I think in each of those cities 90 Euro is reasonable, maybe something with the bath down the hall, but with some investigation, do-able. I would start with some Rick Steves picks, but contact them for an April price, let them know you are flexible, prefer budget (they will push "deluxe" rooms first) You can also use Venere and Trip Advisor to target additional one and two star hotels not in RS; or also consider Lonely Planet and Lets Go picks. In all cases, contact the hotel directly, skipping even their own booking page, to get a price offered by a person rather than a rack rate.
For your trip you might also consider a longer stay in either Florence or Siena, day tripping to the other. Also, if Venice proves difficult, consider Padua or other towns within a short distance, taking the train in.